Use separate pairs of cleats for hard and soft grounds -- three pairs for firm, one for soft. Use the hard-ground cleats when the weather is fair and the field is in good condition. During poor weather or on a muddy field, wear the soft-ground cleats. Separating your equipment in this way will ensure that your cleats are already adapted to game conditions. You may also want to wear separate cleats for matches on turf, as opposed to grass.
Pre-wear your cleats in the shower, and break them in by wearing them in warmups or while jogging. After you get out of the shower, wear your cleats for about 30 minutes to allow the leather to conform to your foot. Before your first game, take a jog in your cleats so that they feel comfortable.
Clean your cleats after games. Remove dirt with warm water and a cloth. Allow them to air dry, outside of your equipment bag. Stuff them with newspaper to help dry out any moisture. Apply boot cream or "leather food" to soften the leather. Store them with a boot tree to maintain their shape. Avoid placing your cleats in direct sunlight.
Turn your cleats into training shoes when you notice signs of wear. Cracking, fraying, and stiffness reduce the resilience of the leather and make it harder to gain traction. A good pair of cleats should last for several seasons.