How not to crash my phone

Today I 'ave mainly been thinking about how to develop an application for my phone without crashing it.  I have no idea what happens when I write an application that locks up. I assume that at the worst I will lose all my contacts :-(. 

My first idea was to use the emulator but sadly I cannot get the python runtime to load into the emulator. The emulator is 5th edition S60, I have looked on the wiki and the Nokia forums but no one else seems to have this problem. It complains about certificates?

I have no second idea.