1. Get your AC unit serviced to make sure it's topped off with freon and also to make sure the condenser is clean. Both will allow for it to run as efficiently as possible.
2. Replace the AC filter. Go with the pleated filters, not the cheap meshed ones. No need to get the name brand though, they're a waste of money.
3. Clear away the vegetation from around the condenser, but don't forget to plant a tree. Trees can help shade your house as long as you plant them on the west or south side of the house. This of course assumes that you live in the Northern hemisphere.
4. Put in some ceiling fans if you haven't already. This helps regulate the temp in the room more efficiently and also allows for you to keep the thermostat at a higher temp.
5. If your AC unit is more than 10 years old and you live in a hot climate, then you should look at getting a whole new unit. I'd wait one more season though to see if you can get it installed in the fall, winter, or spring. AC unit prices have a tendency to go up in the summer time.
6. Strategically open your windows. On cool nights, open your windows and then close them when you wake up. This works especially well if you have a 2 or 3 story home, since heat rises it helps cool the entire house.
By doing all of these things you could cut your cooling costs by half, which is quite a big deal if you have to run your AC unit for 6 months a year like me.