Safari seems to be the most common app that crashes, and the easiest way to reduce this is to go into the Safari settings and empty your cache and history and cookies from time-to-time. It also is good to leave some empty room in the memory, and to periodically shut the device down and then reboot.
If it is an unhacked iPhone, the biggest problem seems to be, as the previous person suggested, memory issues related to bugs in Safari—especially once you have more than two tabs open. Large videos in the iPod section can also result in memory issues.
And if you’ve hacked it, well, that’s a whole other topic…