Baz wrote:

How can I do this easily and without too much mess?

Here's what I do, and it's a variation on what's done commercially:

Scoop the pulp and seeds out of the tomatoes and mix with an equal volume
of water; put in a jar for 2 or 3 days (if longer, some varieties may start
to sprout). The pulp will start to ferment, which helps break down the gel
around the seeds. Swirl the and shake the jar, then let stand a few
minutes. The good seeds will sink to the bottom. Pour off the pulp and
crud and add some water. Repeat a few times and you should have nothing
but clean seeds on the bottom. Spread these out in a fine sieve to dry; a
small fan speeds things up. You can break the dry seeds apart by rubbing
with your fingers.
The whole process takes very little actual work time, and you get nice
clean non-messy seeds. The fermentation is believed to at least discourage
some disease organisms.

--
Gary Woods AKA K2AHC- PGP key on request, or at home.earthlink.net/~garygarlic
Zone 5/4 in upstate New York, 1420' elevation. NY WO G