Here's a video tutorial from Clifton Middleton on saving tomato seeds. His thoughts on saving seeds in general are pretty much the same as mine. The video is 5 minutes long, if you can't view this gardening video here are the hightlights:
- When choosing tomatoes for seeds choose the reddest and ripest.
- Some of the seeds are green and some are yellow.
- The green seeds are "not good" and they won't germinate.
I also have a post with pictures and information on saving tomato seeds.