Take a short (5-10min.), cold shower whenever you feel too hot. It lowers your core temperature which means you'll stay cool long after you get out of the shower. This is great when you come home after work and first walk into the house. Plus you'll be undressing anyway (see step 1) so why not cool off a bit more while you're at it?
This is also great right before you go to sleep since the body must lower its temperature before you can fall asleep
at night. I fall asleep much faster this way in the summer.
If you don't like the shock of stepping under cold water, you can try a cool shower (which works just as well but takes a little more time) or you can start off with a moderate temperature and slowly reduce it every minute or so. This has its own relaxing quality, much like if you were slowly raising the temperature.
Make sure to get your hair wet; that will keep you cool even longer.
If you are following step one and do not need to put on a bunch of clothes, then there's no need to dry off. The water on your skin will act like sweat and carry away your body heat as it evaporates.
BONUS: A cold shower uses less electricity than a hot shower. Saves money & carbon emissions!
BONUS: Your house will not get full of hot steam, which raises the temperature.
BONUS: If you like long showers, you won't use up all of the hot water no matter how long your shower is.
EDIT: Several people have suggested getting your clothes wet and then putting them on to stay cool. I don't do this because I don't like having heavy clothes hanging off of me, slowly me down, getting my furniture wet. But if you don't mind these things, then go for it. It will indeed keep you very cool.