Mr. Lilac is a furnace/AC guy for a living. He said you can clean the outside unit yourself, but shut off the power first and "don't beat up the unit", for example using too high of a pressure of water and bending the fins. Things like checking the level of the coolant is, in our area, only to be done by someone with a licence. The gauges are somewhat expensive, you need to know what you are looking for, and most importantly, if you do it wrong, you could release harmful gases into the environment. He said it is also hard for the layperson to check the electrical.
Best thing you can do is make sure to change your filter every month. (We have a washable filter.) If you don't, it makes the furnace work harder and can cause the motor to burn out. He also said the really fine filters aren't worth it, but to buy the accordian type filters because those fibrous ones don't keep enough crap out of the furnace. (he recommends the red Filtrite or similar.)
Basic maintenance is the best thing you can do in his opinion!
