I think the biggest things are:
1) Keep the BT radio on all the time. Some people report issues with stability that get averted if they do this.
2) Read
this thread and consider following the directions. Another thing that seems to improve stability for people.
3) Aside from these two things, be very careful at this point, sadly, about installing software, even things that seem innocuous. Be prepared that you may run into stability issues, and may not even be able to solve them by uninstalling, with some software, and may need to hard reset. In particular, be very, very afraid of plugins for the Today screen. If you can find multiple people using it on Aximsite, it's probably okay. This is a temporary precaution.
4) Check out the threads here about modifications to get VGA mode some / all of the time. With Adobe Acrobat, it is absolutely breathtaking. (P.S. Acrobat is safe for the x51v)
5) Get TCPMP (also safe for the x51v), because it rocks. :) Or possibly also Mortplayer, which people like here. But TCPMP is definitely safe and is wonderful.
(3) is a hassle, but I love my Axim, and I'm really happy with the purchase, and it will pass.
I hope some of that helps? Otherwise, say what you want tips on, and I'm sure people will be glad to help. :)