I've bagged tomato blossoms using the bags as shown in the link. IME bags larger than those are better--once the tomato plants get growing, they tend to engulf the bags and I lose them. Sometimes I don't find them again until I notice a malformed ripe fruit or cluster jammed into one of the bags.

Make sure you apply the bag before any blossoms on the cluster have opened. When all on that cluster have set fruit, you can carefully remove the bag. Tie a bright-colored length of yarn to the stem of the cluster to identify the fruits as ones to use for seed saving.

Start really early in your season. Excess heat combined with the organza bags shown in the picture can prevent blossoms from setting fruit. I sometimes buy tulle, that mesh used for ballerina tutus, and make bags myself. The air flow seems to keep the pollen from overheating.

Good luck!

Thanks for that link! I am growing Tomatoes and keeping seeds this year, so I was looking for this info too.
Robin

Bitterwort, thanks for the advice. It's nice to hear from someone who's done this. I'll be on the lookout for bags or other breathable material.

Robin, I am really looking forward to saving veggie seeds this year. I saved a few flower seeds this past year for the first time, and that was a lot of fun. My husband's going to love it when I have plates of various seeds drying all over the house ;)