There are coil cleaning detergents that you spray on and rinse off. Or you can blow it out with compressed air.

This post was edited by Geoffrey_B on Mon, Mar 25, 13 at 14:21

The coil cleaning detergents are very effective. They have different types (for indoor coil vs outdoor coil) Use caution as they are acidic (especially the outdoor solution). Warm soapy water, followed up with a warm water rinse down is effective also. Compressed air is a great follow up after utilizing the coil solution or soapy water first.

Here is a link that might be useful: A/C Cleaning