For a while, I've been using this double sided pressure sensitive adhesive that is generally used to apply photopolymer letterpress plates to an aluminum base of some kind. It can be had from a number of suppliers.
It comes on a roll and since I process those polymer plates myself at the shop anyhow, I have it on hand and usually have a few extra square inches or strips laying around when I need it. It's about 1.00 a square foot, anyhow, cheap enough not to worry about and pretty durable/re-usable.
You stick the adhesive to the same paperstock you're printing with (or whatever else you want to use that is plastic or paper and flexible), just a couple of squares of it- and these become your register tabs.
The best part about this stuff is that it stays in place nicely if you lightly apply it, but is repositionable. If you burnish it down it'll outlast your run easily. You can use a bit of it on top of the guides to make a finger/flap which will keep your sheet from popping over the guides as well.
Usually, I'll cut a strip of it and stick it to the edge of a scrap sheet, then chop a few 2X2 inch squares down really carefully with the guillotine.
(Same stock = no squeegee abrasion problems as andy noted.)