If you're thinking of physical comfort in hot weather (rather than fashion, which is a subjective matter) -
Big knickers prevent irritation from low rise underwear rubbing directly on hot and damp section overhangs. If you get high waist (over the navel) antichafing shorts, they are more comfortable and protect the skin over a wider area. If you're prone to sweating, adding a pure cotton, unperfumed towel or liner also helps, as does trimming any hair.
Bras with thicker straps, wider back band (make sure you get a back size that doesn't slice into you, particularly if you normally wear underwired) and less scratchy lace reduce chafing there.
Using a long acting cream/stick antiperspirant on skin folds, upper thighs and the small of the back is easier on your skin.
Physical coverage is better sun protection than sunblock. You don't have to expose everything - and missing a bit with sunblock or running out because you're having to cover from neck to cleavage, upper back, shoulders and full arms every day is painful. Obviously, you still need sunblock, but at least it's at sustainable quantities, as you're covering face, neck, nape of neck, hands, not 1/3 of your body every morning.
Dresses are cooler than trousers or a skirt and top - the need for the waistline to be tighter on separates creates another area for heat/discomfort, whereas a dress doesn't do that.
Unless you're wearing supportive, comfortable sandals, you're going to need a way to reduce the sweat, slippage and friction on your feet - so thin socks or some kind of liner in your shoes is important. Using larger wound dressings at the first sign of a hotspot on your foot rather than plasters massively reduces the likelihood of blisters, cuts, swelling and a shedload of pain. Slip on synthetic shoes are not your friend in the summer.
Essentially, once you've taken care of your physical comfort at skin level, half the battle is won; all you need to is pull a dress over your head/step into it, put your footwear on and you're winning.