A clean air conditioner makes a bigger difference than you'd think. Here's the most effective method, plus the mistakes most people make.
Always turn off the air conditioner at the breaker before cleaning to prevent electric shock.
Remove the filter and vacuum it. If washable, rinse with water and let dry completely. Replace if disposable.
Wipe the front grille and panel with a damp cloth and Multi-Surface Cleaner. Let dry.
Use the vacuum brush attachment to gently clean the coils. Be careful -- the fins are delicate and bend easily.
Wipe the exterior with a damp cloth. Reinstall the filter and reassemble. Restore power and test.