It's hot. Parts of your body are sticking to other parts of your body, and it's horrible. Lucky for you, you've got air conditioning. Unlucky for you, A/C is a real punisher on the power bill.
Here are a few tips for maximizing your cool without crippling your bank account.
Don't Be Greedy
You've been sweating your face off outside, and nothing feels better than coming in to a nice, cold room. Your impulse will be to set your A/C to 66 degrees and let it get nice and nippy. Don't. Keep it set to 78 degrees. You don't need to be cold, you just need to be comfortable. For every degree below 78, you are increasing your energy usage by approximately 8%. No bueno. You don't need to be indoors in a sweatshirt. Keep it at 78 or higher and rock a t-shirt and shorts. If you have a window unit with Low, Med, and Hi as your only options, keep it as low as you can handle.
Divide and Conquer
It's better to use your A/C minimally and have one cool room than to have your A/C maxed out and have your whole apartment only marginally less blistering. If you don't have a door that you can close between rooms, improvise! Hang a thick blanket in between rooms to create a nice cool-box that doesn't stress your A/C. (I recommend using picture-frame hangers, and then threading a few safety pins into the blanket so it's easy to put up and take down.)
Ceiling fans use waaaay less power than an A/C unit, consuming about the same amount of energy as a 100w light bulb, which isn't too bad, comparatively. Make sure the fan is going in the correct direction so that it is pushing air downward toward you (the higher edge of the fan-blade should be the leading edge in its rotation). Ceiling fans don't actually make the room cooler, but they make you feel cooler when they're blowing on you (think wind-chill). As with an A/C unit, if you're not in that room, turn your fan off.
Keep It Clean
Your A/C unit has a filter. It's thin, light, easily removable, and if you've never seen it then it's probably pretty disgusting right now. Pull it out, stick it in the shower, and give it a good once-over with a sponge. Let it dry then put it back in—and be thankful you aren't breathing that crap anymore. This will increase the unit's output and lower the temp. The other half of this is keeping the coil clean. If you're on the ground floor, then it's pretty easy: just use a garden hose to blast some water into the unit through the grates at the top and the sides. If you're on an upper floor of an apartment building, you're going to have to get creative. Try using a portable, pumpable paint-sprayer (but use water, not paint… duh).
Slickaslicka Slim Shady
This one's a twofer. Keeping your air conditioning unit in the shade can make it up to 10% more efficient. That's pretty major. If you have means to build a shade for it (that's not too close so there's no vent blockage), by all means, do it.
The other half of this is shading your apartment. We covered some of this recently in our article on low-budget cooling, but this especially applies when using A/C. When it's hot and sunny lower your blinds, yes, obviously, but if you really want to see a difference get yourself some honeycomb blinds (also called celular shades). They are fantastic insulators. They will help keep the cooled air in during the summer and they're great for the winter when you want to keep the cold air out (and they can darken the hell out of a room for a more cinematic movie viewing experience). They will reduce your bill and start paying for themselves very quickly.