I'm better with cold/wet weather tips than hot weather tips. Sadly I've got much more experience with the cold and wet conditions.
Things like ...
-- Splurge on a good waterproof-breathable cycling jacket with lots of ventilation. You will not regret it! It'll keep you warm and dry both on the bicycle and off.
-- Get a helmet cover with a bill, and a neck flap. It can also be used on and off the bicycle ... while you are riding, and while you are frantically trying to set up your tent in a torrential rainstorm.
-- Pick up two or three sets of those little mini-gloves which you can get for $1 in Walmart during the winter. They are small so they don't take up much room, or add much to the weight of the load. Wear them under your cycling gloves in colder or wet conditions.