Looking through pictures of those beautiful lithops plants grown in greenhouses by professionals, with their perfect sizes, colors and shapes, always makes me wonder if it is possible to achieve that at home on a windowsill. Surely the conditions are different which would affect the process of growth as well as the looks of the plants. But there must be a way to adjust the care to the particular growing conditions and keep the plants not only healthy but also authentically looking. Compact, flat, growing near to the ground, richly colored - that's how we all like them. Reading online about other grower's conditions and looking for tips in books helps a lot, but there's still a lot to figure out on your own, with the conditions and care you are able to provide in mind.
The problem hobby-growers like myself encounter first are plants growing too high, as if looking for the light. It's okay if it is just a little bit, and there are species for those it is absolutely normal, but it can get out of hand and end up badly for the plant (they burst and rot). Basically, the reason they do that is too much water and "food" with too little light. And knowing what 'too much' and 'too little' actually means isn't that easy - trying things out and watching the plants helps to determine what would be the best. Usually it means no water at all for grown-up plants from ca. October until the old leaves have completely dried out sometime in the spring. This way the new leaves can grow and emerge safely, without any unwelcome growth spurts. By the time the change is complete there should be enough natural light for the plant to not grow in the upward direction when watered. The long dry period may seem harsh but lithops have large reserves in the old leaves to bring them through winter. In fact, if watered regularly while the old leaves are still there, they will never dry out and will probably start to rot soon enough. They say that low temperatures in winter are important but I've never tried that (it is also impossible to do on a windowsill) and the plants don't seem to complain.
To keep the new leaves "in shape" it is important to not feed them too much. Normally the lithops soil is very poor (if bought in a store they should be transplanted immediately), which makes it easy to control how much fertilizer to use. When in doubt it's always better to leave it. Same goes for watering. In the hot months you always see if they need more: they are getting wrinkles (small ones don't count: they'll go away the next morning without any moisture) and try to dive under the stones to escape the sun. This is how it should be. If they do that watering once a week shouldn't be a problem. During a cold rainy summer watering once a month or even less would be enough. Also, small pots can be watered more often than large ones. :)
Still, even if you watch them closely, adjusting your 'moves' according to every change in the plant's looks, some may grow too big over the summer. The 'correction method' you find online is quite invasive and risky, but I've tried it this year on a plant. Sometimes the plant corrects itself pushing the new leaves through the side. I guess, this method comes from this observation. Basically you cut the old overgrown leaves open to prevent the new ones from having to grow to the same height as the old body to emerge. I had an overgrown helmutii (even for a helmutii), which I've cut open a couple of months ago. The old body is now much smaller, really soft and wrinkly, while the new leaves are getting bigger without growing upwards. So far, so good. The plant won't get any water until the old leaves are completely gone (I will keep you updated on this one).