Just did my first post earlier today (Frey-Daytona boots) so I guess I'm on a roll. Well that post was related to hot weather gear as well.
I live in Arizona so I get my share of hot weather riding. My own personal tolerance level goes up to about 105F and after that there is not much you can really do to be comfortable, even if it is a dry heat.
A couple of weeks ago I rode out to Bill Mayer to have my saddle redone. Of course I picked one of the hottest weeks of the year. Rode out in one day, around 500 miles. Going across the desert it got up to 107F. Took two days to come back and I saw temps as high as 112F.
The gear that I found to work best for me after spending a small fortune on stuff that stays in the closet ....
Riding suit - Earlier this year I bought the Olympia AirGlide 2 suit. I found this to have very good venting and the silver color reflects some of the heat. High quality at a very reasonable price. The liners work well on the rare cool morning. Highly recommended.
Evaporative vest - A must when the temperature gets higher than 95F or so. In the really dry heat it lasts only an hour or two but it only takes minutes to re-soak it. Mine is a TechNiche but I imagine they all work in a similar manner. In low humidity it's amazing how well they work especially with the AirGlide suit. It only works so-so under my Aerostitch Roadcrafter because the 'stitch doesn't vent as well. A must when the humidity is low, but I don't think they work well with high humidity.
Under the suit - This is probably as important or maybe even more important than the outer suit. Get some high tech wicking t-shirts. I got mine from REI and they are made by North Face among others. They work great under the evaporative vest. As for the bottom half, get some biking shorts. I like the non-padded type (after all I just spent megabucks on the Mayer seat). My favorite are the ones from LD Comfort. Sometimes I just wear the bike shorts under the AirGlide pants, other times with shorts and also with jeans between the bike shorts and outer pants. It's somewhat of a crap shoot as to what works best. Additional layers insulate my legs somewhat from the lower engine heat, but block airflow. The ideal would be to have something solid up to my knee and all venting above that.
Gloves - I use my Lee Parks deerskin gloves year around. I have two pair, one with the shorter gauntlets that I mostly use in summer but the length of the gauntlet really does not make much difference.
Sheepskin buttpad - In the heat your butt will sweat and you will become uncomfortable. Even with my fancy new seat covered in leather (which I love), my butt sweats. The sheepskin really does help by absorbing some of the moisture and circulating a little air (not much). I usually don't use it in more moderate temperatures.
Hydration - The most important. If you only have $20 to spend, do it here and let the other stuff wait. I use a CamelBak type bladder in my tank bag. I have an insulated sleeve for it and it stays pretty cold when filled with ice cubes. I can drink while I ride (even with a full face helmet) and you can't believe how refreshing a cold sip of water is when you are hot. In really hot weather I'm sipping almost constantly.
This works for me, but as I said, when it's really hot, it's nearly impossible to be comfortable.