One of the easiest ways to keep any air conditioning unit clean is to replace a dirty filter. Yes you could just clean the old one and I have done this in a pinch, but the cost of a filter is minimal and I always recommend new. As a matter of fact I recommend buying more than one at a time as you will be apt to change the AC filter more frequently. Running your air conditioner with a clogged filter will cause a rise in your electric or gas bill and leave your air with a musty smell.
Another simple check you can do is go outside and take a look at your fan unit. Make sure there are no tall weeds or shrubs blocking air flow to the unit. Poorly circulating air at the condenser will cause problems just like a dirty filter.
If you can take a visit to your attic, pay attention to the duct work. Are there any signs of wear or leaks? If the sheath around the insulation is peeling that’s a good sign that you need to have a trained HVAC professional have a look.
Have a look at your thermostat to be sure it is keeping your home at the set temperature. If you have an older model thermostat, you may want to consider an upgrade to a newer one that is programmable. That way you can do things like set the temperature higher when no one is home and have it set a lower temperature, say 30 minutes before you arrive.