+1 on cooling sleeves (or the bolero if it fits you) and pouring water on yourself at every opportunity, particularly your jersey and your hair that will retain water for a little longer (make sure your phone and car remote are in a good watertight bag!).

Don't forget to replace electrolytes in the water you drink.

Try to minimize your use of sunblocking creams - that keep you from sweating and cooling yourself - and rely on covering your skin with cooling fabric instead.

The colder you can keep the water you drink, the less you'll need to rely on inefficient evaporative cooling. If you freeze your water bottles overnight, they'll take an hour and a half or longer to thaw in very hot weather, and you can drink the ice-cold water as it melts. I've never used an insulated bottle, but some people swear by them.

Other than my cooling sleeves, I'll exclusively wear black shorts and hi-viz yellow jersey, but if you're okay with wearing white, then that will reflect more solar radiation than any other color.

Something I haven't tried but might investigate is carrying a small reusable ice pack. Those things stay cold a long time. I don't know where you'd buy ones small enough to carry, but you might try to mooch a couple from your doctor's office - the little ones they use for shipping medications are just the right size for a jersey pocket.

FWIW, those gel bandanas actually make me hotter. The heat transfer between the evaporative outside and my neck inside is too slow, so all it does is insulate my neck. Same thing with the Zoot cooling hat - I love their sleeves, but since my hair separates the cap from my skin, it creates an insulating layer and is just unbearably hot even at moderate levels of exertion.

An evaporative cooling vest *has* been an enormous help to me on motorcycles, but (1) the gel layer is thinner than what you get in a bandana; and (2) since I don't race, the aerobic effort and heat generation of riding a motorcycle is for the most part closer to a fast walk than it is to a strenuous bicycle ride; and (3) the speeds and airflow are obviously much higher. I've never tried it on the bici because I'm pretty sure it wouldn't do any good below a sustained 35-40 mph ... and not that I'm remotely capable of sustaining that speed on a bici, but for someone who could, I think they'd be generating more body heat than the vest could overcome.