IMO - the most common reason for Revit to crash at our firm is the lack of resources (RAM). Most of our computers have at least 2 GB or RAM and up to 4 GB). The majority of our users have 4GB...
Our PCs range from 3.8 Pentium 4's to Core 2 duo (better) and we're running on XP Pro (32 bit) at the moment, so adding more RAM won't help because of the OS and it's RAM limitation. We do have the 3 GB switch enabled on most PC's, (a few just didn't like this setting so we couldn't utilize..). You can go here is you want more information on this subject.
SOLUTION: 64 bit OS with at least 8 GB of RAM... (now if money only grew on trees!)
If you don't have this as a option, then you can also use the Worksharing Monitor's System Performance Monitor to help catch a low resource issue prior to it happening. As you can see in my System Performance monitor, running a 64 bit OS (Windows 7) with 8 GB of RAM, I have ton's of resources compared to a 32 bit OS with 2GB of RAM. Also, with notifications on when using the Worksharing Monitor, you will get a window popup window letting you know you're running low and had better save.
Windows XP - PRo - 32 bit:
Windows 7 - 64 bit:
Another issue that will not cause a crash, but will prohibit you from saving due to insufficient resources when your temp folder is full. What is full you may ask? I asked Revit support the same question and didn't get a definitive answer, but I can tell you this, if Revit crashes and you don't clean out this folder (see more on this below), you WILL have this problem eventually... My guess is if you get around 1 GB of "stuff" in there, you'll start to have problems.
Revit creates temp files during normal use of the application. When Revit closes normally, these files are deleted. If you crash, they are not... and they can take up quite a bit of space. Here is an example of the files I'm talking about and you can see that they're pretty big. As you can see, this is only one day - imagine if you crash multiple times over different days!
Close any running applications, then go to >Start >Run and type in %temp%. This will get you quickly to the temp folder. Delete anything that is not today's date and all Revit temp files. You'll probably have a bunch of other windows files in there that can also be deleted, but if you're nervous about deleting them, just remove the Revit temp files.
Good luck and enjoy!