The first step is to seal your home as much as possible, as cracks and crevices will only cost you money in higher AC bills in the long run.
Windows are the biggest source of draughts, especially in older houses that may have less efficient frames.
To spot a draught, get hold of an incense stick and light it, holding it near the window to see if the smoke shows up where air is getting in.
Use a sealant gun to fill any gaps in the frames.
Windows that face the sun should be taken advantage of by opening the curtains to allow sunlight to heat the house. Conversely closing the blinds at night will add a little extra insulation.
Keeping hot air out is good, but of equal importance is keeping cold air in.
Your exterior walls are also prime locations for insulation. Cavity wall insulation can be installed for relatively little cost and will make a big difference to the amount of retained cold and as a result, your AC bill.
Windows and doors can also be a liability when it comes to retaining warm air. Windows should be double-glazed and filled with insulation, usually argon gas, between them. To stop draughts getting in through the frame, insulate each edge of the frame and test the seal by holding a match near the window - if it flickers, air is still getting in.
Another area to check on is telephone or TV entry points into the house - if you've had satellite television installed they will most likely have had to drill a hole through your wall to run the cables, so check these are properly sealed too.