Many editors either have had problems with Final Cut Pro X hanging and crashing, or they will have. The good people at FCP Effects recently posted an article suggesting repairing OS X disk permissions as a cure for your FCPX woes. I don't think that's the best advice.
Repairing permissions is probably the last thing you should try. Many, like John Gruber, have called it voodoo, the product of magic thinking: Seriously, ‘Repair Permissions’ Is Voodoo. Resetting permissions is unlikely to do any harm, but it probably won't help either.
Most software trouble with OS X can be narrowed down to corrupt preferences and caches. Maybe corrupted fonts. That's about it. Not a huge list of things to check by any means. And more recently, it seems Apple has changed how many preference files work to remedy that part of things. That leaves caches as the prime suspect.
I have seen IT people who didn't know what they were doing literally pulling software systems apart for days - mistaking OS X for OS 9 or an earlier version of Windows - in response to a familiar problem with corrupt font caches that takes 5 minutes to fix. If these people start manually messing with the permissions settings on your files, you should definitely repair permissions. But other than that, perhaps save it as a last resort.
A Safe Boot is a better place to begin. Start up while holding the SHIFT key. Hold it until you see a progress bar under the grey Apple logo. This will clear lots of caches. Remember to restart after you Safe Boot. You want to clear caches, not operate in Safe Boot Mode. A lot of stuff doesn't work in Safe Mode. FCPX won't even launch. Let that be your reminder: you forgot to restart. Read what Apple has to say about it: Mac OS X: What is Safe Boot, Safe Mode?
If a problem is easy to reproduce, try the most fundamental and elegant trouble-shooting maneuver at your disposal: log in as another user and try to reproduce the problem. If the problem disappears, this means the cause of the problem is in your Home folder. Leave your poor Mac be. It's not the computer, it's your stuff. Check caches and preferences in your Home/Library folder (now hidden in OS X Lion - see how to reveal it). Check in Application Support.
If you can confirm the cause of the problem lies in your Home folder, move Final Cut preference files and caches to the Trash. When you re-lauch, the app will create a fresh set of preferences. You will likely lose settings in the FCPX's Preferences window, but FCPX has so few Preference settings they are easy to reset.
You should certainly use Disk Utility to verify your startup disk if you are having problems, and repair it if needed, especially since Apple have made it so damned convenient. After decades of rebooting from another disk to check and repair a hard disk, you can now do it without even logging out. That does seem like magic.
And check your media disks while you are at it. But beyond that, there is not very much you can do, really. If you ever see a dark curtain descend over your screen with Japanese writing on it, that's a kernel panic, which is a proper system crash, most likely caused by dodgy peripherals. Check your drives. Swap out cables. Test particular ports on your drives and computer. Disconnect things.
I wonder if most problems with FCPX are not simply the result of throwing way too much processing at your Mac and relying on Background Processing. I can attest this will cause constant beach balling - give it 10 minutes, it will recover - sluggishness and crashing.
This would suggest editing with ProRes and avoiding as much rendering as possible is probably worth a 1,000 disk reparations. This is probably the very best thing to do to increase stability and speed and reduce problems.