First buy quality reels. Make sure that they are rated for saltwater use. I have good luck with Penn SS series spinning reels. There are other quality saltwater reels out there.
I generally just hose them off with fresh water on a spray setting each and every time I use them. I have buddies that wash them down with soapy water after each use. The key is don't use a sharp stream to wash them down as it will tend to drive salt into the inner workings of the reel.
I sometimes will wipe them down with a rag that has WD40 on it. I do this on my (1) calcutta reel on a regular basis. I avoid spraying the line with WD40 as it can not be any good for the line.
Once a season take them apart and grease them up. I am a 10 to 15 trips a year kind of guy and have different rods for different kind of fishing. So I guess that if you go way lots, that this would translate to every 10 to 20 uses consider breaking them down, inspecting them and apply appropriate lubricant to the gears, etc.
If a reel takes a swim in saltwater or lands in the sand it is in for a breakdown and clean before the week is out.