I always require a 25% upfront cost of the total quote, then the 75% upon delivering the final application. There have been special cases where I have setup payment plans for my clients; however, I hide fail-safe bits of of code in the application to which will cause the application to fail after a certain date if payments are not made. Should the full amount be paid, before turning over the code, I remove all bits of this code. This rule is simple for me, if everything is paid in full, nothing is left behind. Don't pay and you just earned yourself a dead program.

Some things I always do:
1) Requirements document is a must. User must sign off on the documents before I even begin so that he/she knows what to expect as delivered for xxxx price.
2) Bug fixes are always handled and corrected through user acceptance testing. All found bugs are fixed as part of the original quote price (obviously if its buggy, I didn't deliver my end of the contract).
3) Upon signoff by the user of user acceptance testing, absolutely no more work or bug fixes will be handled. However, if changes are required or if something needs a few more additions, I have no problem creating a new requirements document focusing on changes (or addendum to the old requirements; either or)