Adequate base preparation is vital.When your preparing your base make sure you have a good six inch layer of gravel.Also just as important is tamping it.You can tamp it by hand or rent yourself a power tamper if the area is large(believe me it's well worth it!!)A poorly tamped base will result in concrete that cracks or shifts.Also place wire mesh resting on bricks(tied).Check your codes there for wire mesh requirements.The wire mesh helps reinforce and prevent cracking.And when you pour before you finish trowel and float,Use a shovel or a rent a "jitterbug" to "work" the concrete,it removes air pockets which can lead to weak spots and cracking.Also if your patio is large enough use expansion joints at required distances.One final suggestion,after your done finishing,keep the slab wet while it cures that also prevents cracks.