I think I posted a How-Too in the seed forum and/or in one of the stickies at the top if this page.
The "Quick" method which scours the seeds using household cleanser actually takes more hands-on time, and may not be the best for long term storage.
The "moldy" method also takes a bit of time and is a bit disgusting. It is best used when processing lots of tomatoes. It is as described below except you do not rub the seeds in the strainer and do not add as much water. The extra solid material enables the mold growth.
For a few tomatoes the least disgusting, least time consuming is as follows.
Get a covered container or ziplock bag, wire strainer, and rubber spatula. Halve the tomato and squish the seed/gel into the strainer. Squish the gel through the strainer and into the container using the spatula. Rub the seeds against the strainer a bit to break up the gel. Add seeds to the container and add water to double or triple the volume. Add a tag (peice of notecard or paper plate w/ pencil). This process takes about two minutes from the time you pick up the tomato. Let sit 4-5 days, check for gas build up every day and relieve pressure if necessary (or simply put the lids on loosely).
The seeds are ready when they quickly clump tightly together after swirling, which indicates the gel sack has decomposed. This may be as soon as 3 days at room temp, but leaving them a couple of days longer is a safe bet.
With this method you will not have a mat of mold on top of the mixture and all you have to do is pour the seed solution into the strainer and rinse (another minute of your hands-on time). The smell is a little sharp/sour, but not too bad and it is down the drain in seconds. Then dry the seeds on a paper plate for 3-4 days. you might want to write the name of the variety on the paper plate.
You do not need to clean the seeds in order for them to grow, just for easy handling/storage and trading. If you just want to plant the seeds next year and avoid the fermentation step I would rub them a bit in the strainer and then dry them spread out on a peice of plastic so that you can scrape them off easily. Store in a pill bottle/little ziplock etc.