You need a source code control methodology and with it you'll get the benefits you're searching for. SVN and other solutions in this space offer "conflict resolution" to avoid inadvertent overwriting/toe squashing.
Setup a subversion repository, get all of the developers up to speed on svn and using it.
Once you have your source under control you can consider setting up a continuous integration server which can build your code and deploy to your target environment in batch. Organizing your project code properly into trunk, tags and branches per solution will make it very easy to control what is deployed or redeployed to your dev environment at any given time.
There are other options for source code control (git, tfs, and many others) but they all offer close to the same features... SVN is one of the nicer options because it's open source, free and stable.
Another thing to consider is keeping your database schema changes in sync with your code changes. Consider using migrator.net or similar solution to enable your team to keep everything in sync through revisions, including database state.