I've been avoiding adding my Aperture libraries to my CrashPlan backup sets because of the sheer number of files in the packages that don't need to be backed up. CrashPlan's memory use goes up with number of files and can increase the chances of crashing.
Luckily, CrashPlan allows the exclusion of files via regular expressions. Quick Googling unearthed an old post by Leigh McCulloch where he shares an expression for excluding thumbnails. By simply replacing "Thumbnails" with "Previews" I added a second expression to exclude those, as well.
The final expressions:
Now I know next to nothing about regular expressions, so I'm sure there's a way to combine them. If there is, please share and I'll update. In the meantime, this works great.