Energy Star Certified- whether you have a central unit that cools the whole house or a window unit to cool each room, make sure that it is energy star certified. It can save you over $100/year. In addition to it being energy efficient, make sure that you are cleaning the filter regularly.
Curtains- an inexpensive detail can make a big difference in your energy costs. Insulated curtains give your windows extra protection against the heat. By blocking out the hot summer sun, the curtains help to keep cool air in.
Insulation- many older homes have insulation that may not be as efficient as the newer types of insulation currently available. Leaks and gaps in insulation can cost a lot of money both in the hot and cold months. You don't have to completely replace the insulation, it may be able to be salvaged by sealing it or adding extra insulation where needed.
Windows- Windows let in the sunlight, but they could also letting all the cool air escape. Energy saving windows can be the upgrade you need to save money on your bills this summer. If replacing them is not in your budget, consider making other changes such as re-caulking and weather stripping to eliminate any areas where the cool air may be getting out.
These small changes will not only keep you cooler and save money on your energy bills this summer, you will also be happy you made these changes once winter comes back around.