User’s of Apple’s iPhoto software will notice that the responsiveness of the application is a function of the number of images (or size) of the photo Library. Once my Library contained over 1000 images, the application began to slow considerably. iPhoto Library Manager to the rescue.
iPLM (I really don’t want to type that long name again) allows you to create and switch between multiple iPhoto Libraries. The documentation suggest topical Libraries — vacation, family, etc. My preference is chronologically dated Libraries. Once a Library grows to about 650 MB, I’ll back it up to CD, and then create a new one (leaving the old one in place, in case I need at any time to switch back to it.)
Additionally, a small shell script run from cron can notify me by email when my current Library is getting close to 650 MB.