Add overhangs to your roofline, the roof should extend nearly 3 feet out from the wall to properly shade the concrete block structure in warmer climates. These can be simple canvas awnings that you remove in winter, or you can extend the roofline using more traditional building techniques.
Shade every window. If possible, replace windows with low-e glass that reflects most heat energy. Add drapes to prevent additional heat from coming inside. Add awnings if direct sunlight comes in the window at any time during the day. Shades should be drawn closed during the day.
Ceilings or roofs should be insulated to at least R-30.
If the ceiling is insulated and the attic is not, the attic needs to be very well vented. Add ridgeline, soffit, gable, and dormer vents. Consider powered venting to ensure the attic stays at air temperature, despite heating from the sun. While there are many types of venting, and some may have particular advantages depending on your location and wind patterns, you almost cannot have too much attic venting.
Use highly reflective roofing. White shingles or roofing sheets will absorb less heat than darker colors.
Trade all incandescent bulbs for LED (or CFL if LED is too expensive). Incandescent bulbs emit a lot of heat. Locate the hot water heater outside the cooled portion of the house. Make sure the dryer vent is clear and sealed along its path to the outside.
If on a concrete slab, remove wood and fabric flooring, and replace it with tile or simply refinish the concrete. The concrete should act as a heat sink to the ground, and without the insulating effects of many types of flooring it should cool the building.
Insulate the outside of the concrete block wall. As others have pointed out, the concrete blocks store a significant amount of thermal energy. By preventing them from getting hot during the day (insulation, shading), then blowing cool night air through the house overnight, they will be able to maintain much lower temperatures than they are right now. It's a problem of averaging - they maintain the average temperature of the building, but right now being exposed to sunlight all day the average temperature is very warm. By reducing sun exposure you bring the average temperature down. You may need to use a dehumidifier during the transition from cool night air to warm humid daytime air to prevent condensation depending on conditions locally.
Make sure your bathroom fans operate and vent to the outside, and use them during and shortly after all showers.
These aren't in any particular order, but I'd put ceiling/attic insulation, roof overhang/awnings, and windows at the top of my list if I could only do a few things. The best way to avoid a hot house is to avoid the heat in the first place.