If it is really PMLE, it's not what a doctor would call heat rash.
What most doctors will call "heat rash" usually happens in areas that are covered by clothing, or in places where the skin folds, like the neck or inside the elbow. Babies get it very often. It normally happens when sweat cannot properly come to the surface of the skin and evaporate. It's very similar to diaper rash. Ointments like A&D or any diaper rash ointment will help with ordinary heat rash. Probably the most common place for an adult to get it is the inner thighs.
PMLE always happens on exposed skin, and since it is an allergic reaction, the best way to prevent it is with an oral antihistamine combined with UVA sunblock, along with "hardening" the skin against sun exposure. As I said in my earlier post, my dermatologist prescribed Claritin, as it doesn't make you sleepy. (Of course, it's available OTC, now.) That combination works very well for me; I haven't had a PMLE eruption in 2 years, and I can spend a lot more time outdoors in summer than I used to.
Here are photos for comparison:
This is PMLE:
http://www.dermnetnz.org/images/pmle.jpg
This is heat rash:
http://www.nhsdirect.nhs.uk/SelfHelp/conditions/heatrsh/_im006.asp
With PMLE, the skin around the bumps usually will not get very hot or red unless you have been scratching. (Which is hard not to do, of course, the bumps itch like mad.) Notice that the person in the PMLE photo is very pale, with lots of freckles. Northern European skin types (esp. freckled readheads, which would be me) get it more commonly than anyone else.
There are several other skin conditions caused by sunlight, these are not the only ones. They don't all respond to the same kinds of treatments or preventatives; if you have a problem with this sort of thing, it's best to see a dermatologist about it. Most internists are not usually very good at knowing what responds to what drug, as they don't have that much practice with skin conditions.