I'm doing a 5x5... and since the weight stays the same for each set, if I get distracted, I easily loose count of which set I'm on. Well, I discovered a great way to keep my set count straight.
You can buy these at any military surplus store; and some of the better outdoor/hiking stores may have them too.
Basically, they're a bunch of black beads, threaded on a couple strands of green 550 cord (parachute cord). They're used for tracking mileage on a road march or hike... know your pace-count, then slide a bead down the cord every time you hit a set distance.
Anyway... In the gym, I just slide a bead down when I finish a set. Now I never forget what set I'm on! I just loop them on the end of the barbell and voila...
Plus, they're a great conversation piece in the gym... almost nobody knows what they are