Practical approach to keeping jQuery up to date?

Question 1

To actually answer your three questions, here are some things I've done or at least recommend:

Best practices for a smooth upgrade rollout

Have tests. These can be unit tests for your JS and/or browser tests. These should cover at least the most typical and the most complex functionality used within your projects. If you don't have tests, write them. If you don't want to write tests, reconsider. If you reeeeally don't want to write tests, at least have a list of use cases that someone will be able to execute manually.
Make sure all your tests pass before the upgrade.
Read the release notes for every (major) version between the version you use now and the most current release. See also the Removed and Deprecated categories in the API docs. If any of your code uses jQuery UI, also look at those release notes and upgrade guides for the interstitial versions. As you do this, make note of the things you would likely have to change in your codebase (possibly making heavy use of grep).
If your project's current jQuery version is >= 1.6.4, also consider using the jQuery Migrate plugin to further assess the work required.
Decide on which version you want to be your upgrade target, based on the work required to get there, whether your project is using any third-party libraries that require a certain version of jQuery, other factors only you can consider, etc.
Meet with your team to go over the list of changes to be done to your codebase, and divide/assign the work accordingly. Maybe write some scripts or other tools to help out. If you have one, your team's coding style guide / best practices document may also need to be updated. Decide on one-shot (recommended) or rolling update releases, if that's possible+desirable. Come up with a suitable release strategy. (I recommend against releasing the upgrade as part of another unrelated large change to your codebase, so it's easy to roll back if you need to.)
Throughout the upgrade process, continually run your tests. When testing manually, always monitor the browser console for new errors. Write new tests that cover unexpected errors.
When all tests pass, decide on how you want to roll out--if it's a site, all users at once or a percentage at a time, etc. For a library or other project, maybe you'd release a beta/bleeding edge version that you can let your more ambitious users test out for you in the wild.
Document everything you just did so it will be easier for the next time.
[Profit.]

How to integrate upgrades into normal workflow

Again, tests. Make sure you have them. Make sure they're good, maintained, and cover a large portion of your codebase and use cases. A continuous integration setup to automate the running of these tests is highly recommended.
Consider getting your team to create and agree to follow a coding style guide or standard. This will make it easier in the future to search for deprecated function calls or constructs, since everyone would be following similar coding patterns. Tools such as scripts, commit hooks, static analysis utils, etc. to enforce good or sniff out bad coding style might be useful (depending on the team).
Investigate and maybe decide to use a package manager like NPM or bower to manage jQuery versions and other third party libraries you might use that depend on it. (You'll still need to maintain your own JS code and go through pretty much the same process as above.)
Again, once you're past version 1.6.4, make sure the Migrate plugin is part of your upgrade workflow.
Assess what worked from the initial big upgrade process, what didn't work, and extract a general process from this that works best with your current workflow. Whether or not you plan to upgrade every time there's a new version, there will be ongoing maintenance tasks and habits that you'd probably want to keep as general development best practices.

Reasonable upgrade schedule

That's essentially a CBA/risk management question. You'll have to weigh some things:

There should be no breaking API changes within the same major version, so you should generally be able to upgrade to the most recent minor version with minimal effort, no refactoring required. This assumes you have and maintain good tests, which you can run on your projects before you decide that all is well enough for a rollout.
Major version upgrades require more research, more refactoring, and more testing. After the research step, you should do a cost-benefit analysis of upgrading.
This might not matter much, but if any of your projects is a website that has many users, what would be the cost of making all of your users have to download essentially all of the changed JS files on your site the next time they visit it (instead of sticking with the older versions that are probably still cached in their browsers)?
The decision to upgrade should always be a subjective one. Minor or major, you'll still have to justify each time whether any upgrade would be worth it. Always read the release notes. Does it fix a security vulnerability or a bug related to issues that you or your users are currently experiencing? Would it significantly improve the performance of your project (be sure to have benchmarks to verify it later)? Does it greatly simplify a coding pattern you've been using, allowing your code to be written more cleanly and easily? Is there a third-party library you want to use that is dependent on this newer version? Are there third-party libraries you already use which are dependent upon the older version? (If so, are these libraries likely to be upgraded anytime soon to work with the newer version?) Are you that confident in your tests and QA process that an upgrade will take a reasonable amount of development resources and cause no major regressions? Were you thinking of eventually switching out jQuery with something else anyway? Etc.

Of course this is just my advice. There are a few recurring themes in it, and I hope they are clear. In any case, I hope someone finds this helpful!

Question 2

You will always be outdated. Once you are done updating to the latest version, a newer one will come out a few months later.

Unless you are willing to put hours/days/weeks of development, testing and bugfixing, with the possibility of breaking user-facing functionality, you shouldn't be updating just to use the newest way of declaring event handlers. It won't hurt you. And normally this is a risky thing to do. This translates into dev team costs. You already know this. Refactoring, especially when there is no evident risk for the project, is in general hard to justify to managers. And you should double check your thoughts to be sure if having the new jQuery in code that is already working will make any difference.

Now, if you are working on creating new pages in an existing site, you could be including a new version in those areas. But, this will have a consequence: lets assume that you and your team, apart from developing the new part of the site, also have to maintain the part that is using the old one. Everybody will need to be aware of the specific version of jQuery they are writing their code against.

So, to close, I would say something like this. Unless there is real justifiable risk for the project to be delayed or to be technically blocked because of an older jQuery version, you are going to be getting into trouble for breaking something that is already working and will need to put extra hours just to make everything work as well as it was working before.

Anyway, this approach doesn't mean that you could start separating the 'new sections' from the old ones, and using the newest libraries in the new areas.

Question 3

This is worth looking into: https://github.com/jquery/jquery-migrate/#readme

This plugin can be used to detect and restore APIs or features that have been deprecated in jQuery and removed as of version 1.9. See the warnings page for more information regarding messages the plugin generates. For more information about the changes made in jQuery 1.9, see the upgrade guide and blog post.

Question 4

The Twitter guys have solved this problem quite nicely.

http://github.com/twitter/bower

It does what it says on the tin - it is a package manager for the web (and that includes keeping JS files like JQuery up to date)

Question 5

In order to keep up to date in your development tree, I recommend using src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.js" (the full un-minified version which allows for easier debugging)

Then when you go to publish, just replace it with the specific minified version that is in the header comment (currently http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js) This has the bonus of allowing better client side caching and using someone else's bandwidth.

If caching is less of a concern than ensuring that it will automatically get bugfixes for that minor version release, you can use just the major and minor version such as: http://ajax.googleapis.com/ajax/libs/jquery/1.9/jquery.min.js (Note: google doesn't yet have the 1.9 series up; however the 1.8 series is up to 1.8.3) Since these get updated periodically for bug fix releases they don't get cached like the version specific releases

Question 6

In this era we cannot be predictable about the stability of the any software versions. Few years Before the versions of software and services releases after a year or two year. But at this time the versions of the services are updating rapidly and frequently.

So if you are using any service with your service you have to use Agile Development. By this development method you can easily make changes in the new requirements and change the required methods according to you.

And please don't use depreciated methods because they are not suitable for long-time service versions. And make libraries of the used services of library of your own service function that are using other services so that you can easily change them according to your new version.

For example : like you have a method name update_var(); it is calling a another method of other service like $a = newlib::check_update();. Then by creating libraries you have to change the main library of the function of your and the core library of the involved service