Our 12 year old gas water heater keeps shutting off and its in the attic above our garage. One plumber came out and relit it and said that it is because it is hot and humid out and that when the weather is that way he goes all day relighting. How do we prevent this? How do we know if we need a new water heater?

Have you tried replacing the thermocouple yet? Its either that or you have a strong wind that's blowing the flame out, which could also be from a weak pilot. I would start by replacing the thermocouple, here is instructions on how to do it yourself.
http://www.americanwaterheater.com/support/manuals/nat_tc_instr.pdf

Hi MizTex...

Kansas has you headed in the right direction... only issue is that the heater he linked you to is a newer heater, but the thermocouple replacement will be the same on yours... OK?

By the way, extremely hot weather is NOT the issue your plumber presented. In all my years, extreme hot weather has never caused a run on water heater pilots going out! What bunk!

As a rule of thumb at my company if we get a pilot light out we will ALWAYS replace the thermocouple as a first line of defense. We ALWAYS carry universal thermocouples and it usually only takes about 20 minutes to replace one.

If you're handy... replace it yourself. If not, call the plumber back and ask him to install a new thermocouple at a reduced rate as he should probably have replaced it the first vist... ;)

Mark