Peat moss is a pretty economical option (I think you can get a 2
cu.ft. brick of it for a couple of bucks) and will add fluffy-ness to
your slow-draining soil. It's also great at holding water--I think
it's able to retain about 20 times its weight in water. The down-side
it that it also adds a bit of acidity to the soil. Check the plant's
acidity tolerance levels before you do it. However, You can
counter-act the acidity by adding in some lime. I don't know about
how fast it breaks down though.

Another good option is to mix in composted manure. It'll add
nutrients while it decomposes and aereates the soil. The downside is
handling the odor if you are going to use it for indoor plants.

Angello

Thanks, that makes sense. Actually, it wasn't "topsoil", but that $4/
bag mix. Maybe "planting mix" instead of "potting mix"? Again, it
appears mostly organic stuff because it's fluffy when it comes out of
the bag, and for some weeks/months after using it. Would be a good
trick to take plain topsoil and make it look that way. Good point
about sand migrating downward in the pot. Had not thought of that.
Also, the shrinkage, which I figured would be an inconvenience, does
indeed have an advantage in removal from the pot.

I'm thinking peat moss is good for fluffing it up as well, but does
that break down rapidly enough that you'd end up with the same organic
glue at the end of a season?