I wear as little as possible when it heats up, but I don't work in an office environment. I work at a produce stand that is basically an outdoor environment; and my boss doesn't care what I wear as long as it isn't vulgar or offensive (no excessive cleavage, shorts should cover my butt, no graphic tees with profanity, etcetera).
EDITED TO ADD: I live and work in Texas, so we get some serious heat with brutal humidity. I have gotten good at dressing for heat.
For an office environment, look for garments in natural fibers. Cotton, linen, lightweight silk... Even some fibers that I call "pseudo-synthetics", like viscose, rayon, and modal can work in the heat, as they are of natural origin and will allow air flow if they aren't too heavily treated or overly thick fabrics. Some people swear by technical fabrics (the kind of material used in running tights, sports bras, and other workout gear) because it is "moisture wicking"; however, I find that petrochemical based fibers like polyester and acrylic make me feel like I'm being suffocated because they just don't let air flow through and they feel really slimy to me when they're wet with sweat (and they can smell funny even after laundering).
Since I don't know what type of dress code your work environment requires, I can give specific suggestions. Whatever fits your dress code, in a natural fiber, should help you withstand the heat rather well.
