I keep breaking the tips off my presta tube stems I can't pinpoint what I'm doing wrong while inflating my tubes.  I use presta valves and have a "Joe Blow" Topeak pump.  I put the tube stem at 12 o'clock, I unscrew the tip of the tube stem (but not too hard), press to let out some air, place the pump head on, lock it on, inflate to 120 psi (as recommended on the tire), then reverse all procedures, trying to keep everything straight (not wiggling the pump head too much).  

I inflate before every ride but didn't think that was a problem as long as I'm letting some air out before pumping and am not inflating beyond the max. psi.

Still, after a couple weeks (riding about 3 times per week), the part of the tube stem that screws up and down is gone, and a few weeks later, after inflating, all the air rushes out when I take the pump off, then it won't hold air again.

The last time I replaced a tube for this reason, I decided to keep the little nut in place at the base of the stem near the tire (though my mechanic says it's unnecessary) and to keep the dust cap on the stem.  I'm thinking these two items might not be necessary to most folks, but they'll perhaps protect against whatever I'm doing wrong.  

Any other advice?
I also use a Joe Blow on my prestas.  After I had a couple valve stems bend and/or break, I started making sure I only unscrew the stem a little bit.  My theory was that by unscrewing the stem all the way up, it was easier to bend or break when pulling the pump head off.  Haven't had it happen again since.