If you're having trouble with your app crashing, first check to see if Foursquare is down (We hate when this happens but we do have occasional downtime! You can usually see if we are down by checking up with us on Twitter).
If Foursquare is up, make sure that you have the most recent version of our app. We regularly update the app with bug fixes, so make sure you have the most recent version from the app store. If you're still consistently having a problem, make sure you've tried restarting your phone and making sure that all settings are allowed (particularly if you are using a BlackBerry. Check our hints here).