I've been experimenting with various setups for PCB etching, and thought I'd share one of the more useful things I'd found, as I don't see much information about the subject anywhere else.
I've been etching using a mixture of HCl and HOOH (see http://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!–A-better-etc/ for more details) and have no access to a laser printer, which rendered the traditional method of resist application impractical. While thinking of other easily-applied materials, I ended up testing white-out (very poor; dissolves in the acid, porous, not easily removed by acetone), scotch tape (very good, but very hard to apply correctly) and petroleum jelly.
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After cleaning it with Acetone, I spread two different thicknesses of petroleum jelly on my copper-clad board (about 1 mm and 3 mm thick), and scraped away jelly with a toothpick along where I wanted to etch. As an additional experiment, in an otherwise empty section of the board, I wiped off most of the jelly with my finger, leaving just a very thin layer.
After a few minutes the etching was finished. The etch was pretty good; everywhere the jelly covered was protected, and although some of the thinner lines had some copper left they were well-defined enough to be easy to clean up with an exacto-knife (I would recommend using something slightly broader than a toothpick to do the scraping, and to spread the jelly thinner so that there is less chance of residue flowing into other lines). The jelly acted as an equally good resist at all thicknesses, including where I'd wiped most of it off.