Sorry I don't have time right now to make this a full-fledged tip!
If you sync your library with an iDevice, this can save you time syncing and disk space.
This is something that's somewhat-known, but also worth bringing up every once and a while, but for those who don't know - the iPod Photo Cache is a folder in your Aperture or iPhoto library that holds the images in a special format when you sync them with an iPod, iPad, iPhone, etc.
Every so often, maybe a few months to a year, I think people should delete their iPod Photo Cache(s). It depends on the volume of photos that you sync with your device and how often that changes. If you have a set up where you commonly sync only recent events or lots of faces then you'll likely have tons of photos.
My iPad had about 12,000 photos on it before I deleted them all. (I was having syncing problems, and this was the primary reason to delete my cache file.) I also deleted my entire iPod Photo Cache (in the root level of the Aperture library) to save space, it was about 45GB. After reorganizing the photos on my iPad, I now have about 10,000 photos on it. The size of my iPod Photo Cache? Under 9GB! Holy cow, over the past maybe 12-18 I've synced nearly 35GB of photos and things that are no longer on my iPad. I deleted all the photos from my iPad before re-adding them, but that's not necessary unless you change photos so often and are experiencing slow syncs.
There is a catch: The next time you need to add photos back to a mobile device, iTunes will need to recreate all it's mobile files. This WILL take quite a while, especially if you try and do 10,000 photos at once. However, my iPad now syncs much better and I have 35GB of 'new' drive space. :)
I know that going into iPhoto and Aperture library files seems risky, but the the folder iPod Photo Cache can be deleted with no affects on your library.
This is easy, find your Aperture or iPhoto library in the Finder. Right-click and select “Show Package Contents” then find “iPod Photo Cache” and delete it. Don't muck around with anything else too much.