I'll offer up my experiences. I spent a summer in the Sea of Cortez, in which the night time lows didn't go under 100f and the sea water temperature was above 90f for months. It was around 20 degrees latitude, and was extremely hot.
In pure sun like that, you need to keep the sun off of your skin, period. You will be hot no matter what you do. There's no way to walk around in 125f heat and not feel intense heat, but you can do your best to keep from being physically harmed from it.
Hat: In the tropics, the sun is typically at a very high angle so broad hats will cover your shoulders. Umbrellas work too.
Shirt: Long sleeve. I use ExOfficio's Sol Cool line with great success.
Pants: Typically in disgustingly hot heat there are disgustingly nasty bugs to match so as much as it stinks you need to wear long pants. Thin synthetics work good. I like Prana pants. You can roll them up to your knees. Also, this keeps the sun off your skin (rolled down).
Shoes: Sneakers with a lot of mesh. Cheap ones might melt (seriously), but given the ground heat and bugs you can't really play around with flipflops if you're in the back country.
Umbrella. It might sound very un-outdoorsy, but I'm a convert for the silver-lined go-lite clone umbrellas (ultralite trekking models). They keep a tremendous amount of heat off of you, and you can take your hat off to cool down more. I use one any time backpacking in the summer with direct sun (no trees).
Other things to consider are skin rashes, infections, heat cramps, and other super fun heat related issues. Until you've lived in the tropics (without AC) for a couple of years, it's really hard to fathom exactly how brutal the sun is. We started nicknaming it "The Orb of Death".
If you're only hanging out for a few days in the desert at 35 degrees latitude, some of this is no big deal. But if you're in the real tropics mid summer, this stuff really can kill you.