I use a mixture of crushed gravel (not round rock), sand and soil add some water and make it into a consistancy of what we call (sp?) Play Dough. If the soil keeps falling or washing out then it's easier to add it between the crevice when you add the plants. Take a short piece of coat hanger wire (a couple inches) or 3" nails, sharp stick, or some other object and poke it through the plant and into the soil. This anchors the plant into the soil and helps to keep the soil pushed into the crevice.

I don't use peat or potting soil but if you do you'll find it 'shrinks' at the edges and your soil will fall out in dry conditions.

Thanks. Will try it. The nail suggestion is brilliant. I think that will really help.

As a sidenote, Baldasarre Mineo(owner of Siskyou Rare Plant Nursery) says that in crevices where there isn't much soil to support growth, you should use a richer soil mix than one would normally have in an alpine garden.

leftwood. Good point. I don't add richer soil but along the same lines I'll poke extra compost or manure into the sides of the crevice from time to time just to add more nutrients.

I had the same problem I was trying to get the Semps to stay in, I wrapped soil in leaves and shoved it in with twigs and anchored the Semps with some more twigs around it until they rooted in good.