At the moment, I've got java fern basically tied to things with string. Basic cotton sewing thread is fine. Eventually the roots of the plant will grab hold onto whatever its tied to. You can also use staples, rubber bands (make sure to take them off before they start to degrade), twist ties, bits of wire, etc. I've got a clump of anubias nana sitting on my gravel, weighed down with bit of string looped around the plant, then tied to a rock covered over with gravel. Kinda like an anchor I suppose. I'm hoping that by the time the thread falls apart, the roots will have taken hold.
Oh yeah, for the switch over, I've left a couple of my nicer looking plastic plants in the tank until the plants grow in a bit more (at this rate, I might be dead by then, lol). It makes the tank look less nekkid hehe.