If you get low battery life, ie, using 3 aa in a weekend, there's either something wrong with the batteries or the headlamp is being used at high. One aa lasts easily 12 hours, I think 15, depends on the milliamps it holds.
To make it clear and simple, the h51 has I believe 7 settings, which work like this:
1. a: low - low - this is too low to read with I think, and definitely too low to hike with.
1. b: low - high - this is fine for camp use, and will last I think a few days, can't remember the hours, it's a lot. Not quite enough for hiking on difficult trails, maybe enough for hiking on flat trails where it's fairly smooth. If you have very good eyes, you can read with this.
2. a: medium - low - fine for most trail hiking, good battery life, 15 hours?
2. b: medium - high - depends on your eyes, one of these is fine for narrow rocky single track trails going up or down mountains, no problem.
3. a: high - low - this is absurdly bright
3. b: high - high - illuminate the canyon, signal helicopters, blind your friends for a laugh.
3. c: strobe - I never used this, can't say anything about it, but it's I'm sure totally adequate for emergency rescue at night.
You set 1, 2 and 3 for your desired level and it remembers that setting, and you cycle between 1 2 and 3 by pushing the on/off button.
If you used even 1 aa battery during a weekend trip, that would be surprising, unless you kept it on from sundown to late at night at medium for some reason.
I think I set mine to 1 - low-high; 2 - medium - low; 3 - high low. That works well for pretty much anything you come across, night hiking to camp. As others note, the high is really only to situate yourself briefly, like if you have a confusing trail junction or something.
Powerex batteries are also supposed to be very good, I got some from the same place I got my powerex maha ma9000 charger. Candlelight forums were quite adamant about the importance of using a good charger, so I figured I'd get one, they are nice, and let you know how much charge those batteries actually hold too, which is useful, once you see the difference battery to battery you'll understand why using a 3 cell aaa headlamp just isn't a very efficient idea.
I store the h51 in a small silnylon rocksack (dual use, groovy), and wrap the headband around it the long way, so it actually sort of keeps the button from getting pushed, at least that's what I've found, but if you are backpacking, you definitely want to turn the bottom a half turn or so to unscrew it to avoid a drained headlamp, it takes very little turn of the end cap you'll see to make the switch not work.