Well...

1. Ensure the source will always be available, in the required form, if needed:
1. Use CVS so you can rollback the code to any point.
2. Implement proper backup proceedures taking into account everything from media failure to natural disasters.
2. Ensure problems can be fixed in an acceptable time frame.
1. Create a modular design.
2. Clearly specify every aspect of the code.
3. Document it well (both high-level user docs and comments).
3. Take steps to prevent problems in the first place:
1. Design everything in detail first.
2. If possible, audit and use existing tested code.
3. Write tests, then code.
4. Refactor, refactor, refactor.
5. Refactor.
4. Remain paranoid.
5. Pass off maintaining the code.
6. Peace of mind (until the new maintainer bothers you ;)

So yeah, if you want peace of mind you'll have to find another job ;-).