If you're thinking about dropping a series, don't put it on your pull list. Most shops will extend the discount to everything you buy, not just the stuff on the list itself, so browse around and pick up the titles you're not sure about on a month-to-month basis. It makes it easier to decide whether you really want to continue buying or not.
You'd be surprised how much psychological inertia just knowing you'd have to bug your shop guys to scratch it off your list when you might potentially want it again later creates. I'm still reading Catwoman, for example, because it's on my pull list. Really, the material inside was worth dropping months ago; I just haven't brought myself to do it because it's a hassle to take it off the list and it's not really bad, just a little mediocre. You start finding ways to justify keeping **** on your list when you know deep down that you could really do without it.