1. Change the filter.
Central air systems should have their filters changed at least every few months. But if you've let the chore slide to the back of your mind, now's as good a time as any to replace the filter.
2. Clean the vents.
Again, a regular dusting several times a year is a must. But if you've been slacking, do it now. You could also consider having a professional give your air ducts and vents a thorough cleaning.
3. Check to see if you're blocking the condensing unit.
The part of your air conditioner that draws air into the system obviously needs to be uncovered. That means any fold-able patio chairs you've conveniently stored in your A/C closet for the winter need to come out. Also take the time to clean obvious obstructions like leaves from around the unit.
4. Check your freon levels.
Your air conditioner isn't consuming freon coolant, so under ideal conditions, the freon would never need to be filled or changed. But you could have an HVAC tech check for a leak (especially with older models) or another irregularity as part of your annual A/C maintenance plan.
5. Check everything else.
If you're going to have a pro come in to check the freon, also have them check that the electrical controls and coils are in top shape. A professional HVAC tech can also calibrate your thermostat.