Meringues crack when they cool too quickly. Leave them in the oven after baking (with the oven turned off) to slow the cooling process and help avoid the cracks.
Meringues may collapse for a couple of reasons. Older egg whites tend to not hold the air bubbles as well as fresher whites, which can cause them to collapse. A more common cause of collapse, though, is that when the whites are beaten too quickly (on too high a speed) they form big unstable air bubbles, which will later collapse. When you whip egg whites for a meringue, start them on low or medium low speed, and increase it only when they become foamy. Even then, increase to medium before finishing them at high speed.
Recipe: Chocolate-Chip Meringue Cookies