Hey Blokeman!
If your Lithops are doing this do not water them at all:
It sounds insane but watering when the plants are doing the skin absorption will eventually kill your living stones because the old skins will suck up the water and try to stay alive, eventually constricting the new growth. For awhile they will become what is called "stacked" and they can end up looking like this:
See how there are 3 pairs of old and new bodies piled up? You don't want them to do this. I got the above plant (Pleiospilos a Lithops relative) already stacked like this from it's previous owner. I haven't watered it since I got it several months ago and it's finally starting to absorb the lowest pair of leaves, I will not water until only the top pair remain, that might be a year as each body is fist sized!
When your Lithops have absorbed their old body completely and look like this with just a dry husk of the old skin (on the lower left body) you can water them again.
Water them real well with fertilizer water (low nitrogen, high phosphorous like Dynabloom 3-12-12) so they get a good load of nutrients and can get their growth on for summer. After the initial watering for the year only water them when they start to "retract" into the soil. Lithops will stand up an inch or so above the gravel when they are fat with water and pull down to gravel surface when they are dry and in actual need of water or they will wrinkle. It's hard to kill them with too little water but too much is deadly.
Not watering your succulents (mesembs especially) is an exercise in self control or "being mean" by withholding the watering can. But these plants will drink until they split open - they all have a drinking problem because they are accustomed to sucking up any little water that comes by either fog/mist or just a few millimeters of actual rain a year and not letting go of it. In cultivation we can give them a little more by watching how they look. I've never actually popped one but I've made a few so fat they were definately right on the verge of "one more drop will do it". It's only luck that kept them from splitting on me.
Acclimate them to bright light to give them good strong colors and hard bodies, not too much hot direct sunlight or they'll bleach out and die direct sun can get very hot and sort of "steam" the water inside your Lithops and then they're "done". Direct sun in the morning (before noon) and afternoon shade is great cos it's a much cooler brightness all day. If growing under fluorescent lights just keep the lights close (within 6" or so) and day temps between 75 - 100*F and down as cool as you can get at night (40-60*F) makes them happiest . Low humidity is required for all the succulents so don't put them under glass, open air (or a fan!) is very good.
If your plants are in pure peat (mine always come that way), I suggest repotting them into Shultz Aquatic Plant Soil ($5 at Home Depot) which is just small fired clay bits that soak up water and dry up totally in a few days. I don't use any peat for any of the succulent plants I grow, minerals/rocks/gravel/clay only. I add some perlite to the clay bits but it may not really be necessary as the perlite seems to float up to the top of the pots anyway. Top dress with some pea gravel so the clay and perlite doesn't wash out when you water. There is also some rumor that watering over the pea gravel gives some trace elements but that could be an old cactus farmers tale. These kinds of plants don't seem to care about repotting at all so go ahead and do it anytime. I repotted several in bloom and they continued to bloom and didn't abort. Just repot in dry mineral mix and
don't water and you'll be fine. Whenever they show signs of needing water then you can give them an "after repotting" drink. Usually about 2-3 weeks after repotting they begin to show signs of wanting some water.
Also, it's a good idea to pot your Lithops individually in 2" pots (they have small roots). One may need water and the other two do not. You can't give them individual care if they're in a group planting (how they are usually sold). Each color & pattern of a Lithops indicates a different collection locality, some need a tad more water some a tad less. You can't really set up a schedule of watering for the Mesembs (Lithops family) you just have to observe them and determine if they can afford to be wet again or not. If they look "chubby" - no water for them!
Here's the two best sites I've found on these guys:
Lithops Botany
http://www.lithops.co.za/
Lithops Gallery
just about every Lithops variety can be found here so sit down with your plants and look over each pic and you'll probably be able to figure out what you've got.
http://www.lithops.info/en/gallery/images.html