I'd really recommend you to start using a distributed version control system (DVCS), such as Git or Mercurial.
You wouldn't need to stop using Dropbox, but rather turn your shared code folder into a repository that allows you to frequently commit any changes that matter.
If you ever need to collaborate, you can look into hosting solutions like Github or Bitbucket, but you could just as easily share your Dropbox folder with other collaborators.
This way you would get a relatively simple yet powerful workflow.