I think of pyramid routines as changing the # of reps per set based on how heavy the weight is.
I prefer to progress by increasing weight while decreasing # of reps in each set. For example, you can do sets of 9 (20), 7 (25), 5(30), 3 (35). If on the last set, you can't hit even 3, then you probably needed to have worked at a lower total volume. If on the last set, you can get out more than 7, you should definitely increase in weight. If you're ranging 3 to 7 on the last weight, then you're at about the right place. Next time around, you might switch up the volume a bit anyway (eg., 25, 30, 30, 35).
There are other ways of doing pyramids, including variations of drop-sets (going to failure and then lowering the weight/reps, repeat) or working your way up and back down in weight. I like the increase weight/decrease rep method, because I think it is the least injury inducing of the lot. Your muscles get "primed" to go heavier from the incremental change. The difference, though, is that you could obviously have lifted more in the last set if you hadn't already done 3 or 4 working sets before!