Here are a few things to try if your app store keep crashing:
Close down all the apps you have open, then reset the phone (by holding the home button and the the lock button together for 5 seconds)
Go into "settings" and reset all settings from "general"
Watch out though! This will return all settings back to how they were when the phone came out of the box - HOWEVER all your images and other data WILL REMAIN.
All else fails? Plug your phone into your computer, launch iTunes and select the "restore" option when it tries to sync your device. This sucks because it can take quite a while, but it rarely fails (unless there are hardware or firmware issues with your device, in which case in may be time to visit the Genius Bar)
They're usually pretty good there, although some people may have concerns taking their phone to the store depending on what is saved on their device...
