From my personal experience (but here you'll surely find better advices) for a natural bodybuilder 5 x 5 works really really well.
Just do compound exercises like BB Bench Press, BB Rows etc etc
I usually set my personal best (1 rep max), then go back to the 80% of it (this way you get 4 to 5 reps).
Two exercises per workout (i.e. Bench press and BB Curls), others could be Deadlift and Rows etc etc, three workout per week, use the same weights and see how the reps will go up.
Do this for 3 week, the fourth rest (hi-reps sets and light weights), then repeat.