There are a few things you can do to try and limit how much the fondant dries out while you're using it. Firstly (and maybe most obviously) the faster you work, the less time it will have to dry out. You will naturally get faster at this the more practice you get. But also, make sure you have everything you need on hand so you don't have to leave your fondant uncovered while you go looking for something. Secondly, try not to use too much cornstarch or icing sugar when kneading and rolling out your fondant. It's better to use a little shortening when kneading to stop the fondant from sticking to the bench. When rolling out the fondant try to only put cornstarch on your work surface, and only use a small amount on the top of the fondant if you really need it. The less you have on the surface of the fondant, the less dry it will be. Thirdly, if your fondant is particularly dry, you can knead a little more shortening into it, or knead in some food-grade glycerine. Be careful not to add too much glycerine, as it can go the other way and make the fondant sticky.