when that starts happening, you should uninstall a few applications from your device, then take the battery out for a little bit, put it back in and power it up.. I had this issue before as well, and called verizon to get help with it, they told me it is a common problem, but the only way they know to fix it currently, is this way.. I try to not have too many applications installed at one time these days, at least until they send out an update with a fix for this problem (i'm sure HTC knows about this and is planning on fixing it)
It's been about 3 weeks now without that issue, everything is working great!
like gtp said, A backup app is always good to have/use.. I use app manager and Sprite Backup.. App manager is nice, because you can save your applications that you've installed to your SD card, so if you do uninstall it to get your device back up and running, you can quickly install it from your sd card, instead of finding it in the market and downloading it again