Here's what you're looking for.
- Save seeds from an open-pollinated, rather than hybrid, tomato. Seeds saved from hybrid tomatoes won't come true, and there's no way to tell what you'll end up with. This may take a little research. If you're unsure whether the tomato you're growing is an heirloom or a hybrid, a quick internet search or a glance through a few seed catalogs will tell you what you need to know.
- Save seeds from tomatoes that are fully ripe, but not over-ripe. Seeds from over-ripe fruits could already be on their way to germinating or flat-out rotting.
- Save seeds from the best-looking, best-tasting fruits on the plant. When you save from the very best, the resulting plants will be predisposed to having those same desirable qualities.
- It's fine to save seed from plants that have suffered from blight or one of the many wilts that affect tomato plants.