I agree with northernguy that zinc oxide ointment is key to preventing diaper-rash. Desitin is particularly effective (it comes from an old Norwegian recipe, as I recall) because it contains cod liver oil, rich in vitamins and nutrients and emollients. (It also helps give Desitin its marvelously medicinal odor.) I must remonstrate regarding baby-powder; I find Johnson & Johnson's Talc Baby Powder really helps to cut down on diaper-friction and prickly heat (especially when urethane pants are worn over cloth diapers in the heat), and does help with comfort, absorbency, and smelling sweetly even after first wetting. Urethane pants are a must. PUL (polyurethane laminate) pants breathe -- they allow sweat to transpire while retaining urine -- but true urethane pants (mine are high-backed Gary "Euroflex" -- truly outstanding for supple, sexy feel, for durability, for translucency, and for top-notch protection over cloth diapers) will last a long time, will tolerate diaper-creams and baby-oil well, and will please any caregiver as well as any wearer.
I avoid the cornstarch powders (sorry, ol' buddy) because they tend to clump more than talc does and they encourage yeast and other opportunistic infections in some (like me). I use lots of Desitin and talc; Desitin first, with talc dusted over. Smells wonderful and lasts for a while. I find it's no problem to wet and wear so long as my plastic pants either breathe (PUL) or get changed from time to time (Sani-Pants, Euroflex). I find the real rubber pants just a little too hot and sweaty this time of year.
I do enjoy my fine Simplicity hospital-grade diapers in the heat. They are lightweight and airy, with silky blue plastic and a nice trim mat that nonetheless absorbs well. These contain well too, and they are just so sexy! I have even worn them discreetly under nylon swim trunks as a quasi-swim diaper (wet only); they work, but of course balloon ridiculously. I haven't yet invested in the high-tech swim diapers for the beach, but I bet they work well. Stay dry and rash-free! Happy nappies.