Here's a suggestion based on what I do when I'm in this situation. It has worked every time I've asked for it.
Whenever I notice that I am over on my minutes, or text messaging, I call customer service before the end of my billing period. Then you just request to be bumped up to the next package, and that the change be made retroactively for the month. Then you are effectively on the next minute/txt package for that whole problematic month.
In some cases, the reps have even told me they can do it until the end of the current billing cycle, then knock me back down automatically to what I was at - so I don't even have to call in again.
So basically if you keep an eye on it, which it sounds like many of us do, then just check a few days before the end of your cycle, and adjust your PLAN accordingly to avoid any overage fees.
Also, do the math - if the cost of the jump to the next plan is greater than the cost you'll incur in overages, don't do it. For me, this is about 40 txt messages over my limit. So if I'm only 20, I don't sweat it and deal w/ the overage charge because it's less than the difference to bump to the next messaging plan
Hope that helps!