The most important thing you can do is clean and replace your filters. Newer thermostats that are ECO ready can help provide some costing savings with advanced detection sensors and scheduling. Outside of these, shades, drapes, shutters, or screens should be installed on windows that are exposed to extreme sunlight.
Every time your system starts up, it will use a lot of electricity and not produce much cooling. Usually a system that is too small to cool the home is more economical to run but delivers less comfort. Even though it runs nonstop, it will usually consume less power than a larger system that cycles on and off. As a rule of thumb, a unit that is either on or off is less expensive than one cycling on and off.