Okay, seriously, this is absolutely annoying at this point... I got some Ernie Ball Super Slinks and strung up my guitar. It wasn't until about a week ago that the ____ thing broke. This isn't the first, however. This is probably the fifth now, but the strangest thing is that it happened when I tried to put it in Drop B? The one after Drop C.... I was beyond pissed because I'm getting a Schechter Stealth C-1 for Christmas and I don't want to invest another pack of Super Slinks in this thing. I've some old strat my uncle fixed up. And it broke at the tuning peg... or what ever it's called.... And the same thing happened on the one I was using before the strat. It was a First Act I bought off my friend. Had the same prob.
What gauge are the strings? It could be that you are attempting to put string on which are to thick.
Dean RC7X (Bareknuckle Coldsweat pickups)
Ibanez Rg2570Z (Bareknuckle Juggernaughts)
Schecter KM-6
Schecter Hellraiser Hybrid 7 String
Engl Powerball II
Orange PPC412
Line 6 Pod HD500X
there is a lot to consider. Size like amonamarthmetal said. But here's some other things.

Easiest answer. Switch to these three kinds of strings
Dunlop Heavycore
Ernie Ball M-steel strings - they are made of indestructible metals (but are like 10$)
GHS Thick core I believe they are called
they are three options. You keep your specific size but they are built to last longer. Which is better you'll find out. Pure nickle strings have a dark sound I find.

a possible reason
tuning up and down on a gauge isn't good to do constantly. This is how I broke a lot of strings in the beginning. This creates fatigue. Capos aren't a terrible idea to get the higher tunings but they don't sound as good on open strings in experience.

some things to consider
if it's breaking near the headstock - it's the strings fatigue for sure.
if it breaks near the bridge. Check the bridge. Especially if it's a floyd rose.

ways to improve a guitar to avoid this
if tuning stabillity isn't an issue like you can go a week or two without tuning it (Casual use) don't worry about it.

a new nut / string saddles. Roller string saddles on les pauls and strats apparently improve string life. Say for example roller tune-o-matic bridges or the schaller vintage 2000 bridge.

hope I could help.
Thanks for the tip. I'll probably get the Ernie Balls you mentioned and put 'em on the C-1 come December.
How old are the strings, and were they in good shape?
Friend of mine turns his strings black within about three days, and within a week there are (I swear) stalactites and rusticles hanging off them. I'm surprised they don't snap more than they do (and they DO snap), and I'm just hoping he's got his tetanus shots current.

My strings last forever, or they would, if I didn't change them as often as I do. I think it's a body chemistry thing ("oh, uh...all of my guitars are in the shop. Nothing here for you to play"). Most guys participate in at least one of three vices to excess: they drink, they smoke, or they indulge in debauched women ( I don't drink or smoke). Some of the folks that drink seem to have more toxic sweat where guitar strings are concerned.