Check the following:
1. Are the lamps and the ballast matched? Simple check there, see if the text on the lamps matches that written on the ballast. Make sure that you're not using the fancy "energy saving" lamps as someone mentioned earlier. They are designed to work as traditional lamps of a slightly higher wattage - but they DO need dedicated ballasts.
2A. Are you using electronic or magnetic ballasts? If they're magnetic types, do they make a lot of noise? Sometimes that can be an indication that they're on their last legs.
2B. Do the lamps strike up easily from cold, or do they take a long time to start? If they tend to flicker a lot initially, you could have one of two things:
a) Borderline ballast voltage. I had one of these a while back, and it was brand new. The ballast, generally if it's a cheap one, cannot actually supply a high enough voltage to either strike or maintain the arc - therefore the lamp may light initally, then go out again - and keep doing this. Mine would usually strike properly *eventually* after flickering for about a minute. That thing used to eat tubes at...well, about the rate you're talking about! All of them still strike up just fine in a decent fixture, but were effectively dead in that one. The only real solution here is to replace the ballast. In my case I replaced it with one from a CFL actually! And two years on, is still working fine - with one of the "dead" lamps! ...same one I put in there two years ago!
b) Dodgy starter. Quite simple. Replace it. Worth trying anyway, they cost all of about $1. Make sure they're correctly rated though. Using a high wattage starter in a low wattage fixture (and vice-versa) can cause problems. I wholly reccomend the electronic pulse starters (the green ones). They do actually improve lamp life. And will leave a dead lamp off, rather than having it flash annoyingly and potentially overheating the ballast.
3. Check your line voltage. If it's in your garage, it could be an extension to the houses original wiring, as such, might not be of the same quality. I the voltage is either too high or love, it could be causing problems.
4. Check wiring. Many fluorescent fixtures will behave oddly if they aren't grounded correctly. The metal frame/reflector is often part of the starting system, providing a capacitive route to ground, assisting in the initial ionisation of the mercury vapour/argon gas fill in the tube.
If that don't sort it, replace the ballast or fixture.
..Failing that, replace the electrician!
Hope that helps.