I always (for the most) start with drums wich the mpc does the best, it is a solid foundation for the tempo and rythm.Next I might add some atmosphere, like a rhodes piano you know the one where the sound goes from one volume, then goes lower, then back higher.From there I might do the bass line, or maybe some strings.I feel the times I make my best beats is when its spur of the moment.Example I was gonna watch a show, but I had an urge to just fiddle with some sounds for just a few minutes.If I say Im gonna go home later on and make beats it may not work, it happens when you aint really trying to make one and your just fucking around with sounds you normally dont use