Buy a handheld fan.
Seriously, I don't know what I'd do without mine! It lives on my desk in the office most of the time, the air conditioning's not so great in here! But it's invaluable on public transport, especially on the metro, it gets so hot in the summer! They're so easy to get hold of too, from people selling them on the street or on the underground, and I've seen loads selling them for 2 euro for a pair! Barganious!