How to stop the development spec from changing in mid development?

Question

Problem: It seems with almost every development effort I'm involved in, no matter how much time is spent planning prior to starting development, there is always a large amount of changes required either midway or towards the end of the project. These are sometimes big changes which require alot of re-development.

I don't work for clients who pay money, this is an in-house development team on in-house development websites. So, it's not like I can charge for it or anything. And at the end of the day, we have to try to hit deadlines.

Questions: What are some of the best ways you guys have found to minimize and prevent spec changes from cropping up midway or after development?