So I kicked my first Corny keg and need to get the keg and the system cleaned and ready for another batch, which might not be ready for some time. What's my process here?
I like to rinse well, then soak in warm PBW (or cleaner of choice). I fill the keg with cleaner, put on the lid and shake really well. Then I remove the lid, poppets, O-rings and dip tubes and put them in the keg to soak overnight. Next day everything gets a good rinse in warm water. I fill the keg with some Starsan (or sanitizer of choice) and let the parts soak for a couple minutes. Reassemble, seal the lid and shake really well. Let it sit for a few minutes.
Then I hook it up to the gas and beverage lines and blow out the starsan leaving the keg sealed. Then I just leave it like that until I am ready for the next beer.
Any remaining foam in the keg will settle out as liquid. When ready to refill, open the top and I pour out that liquid with a bit of a swirl to help resanitize the opening at the lip a little bit. Then rack beer in.
The key to kegging happiness, IMO, is having empty kegs cleaned and sanitizes right away. Then you don't have to go looking for cleaner and sanitizer the day your next batch is ready.
First, shed a little tear for the empty keg. Then get excited about the next beer to fill it.
I'm not as methodical as brewchez. There are four dirty kegs sitting open in my garage right now. As you can imagine the leftover beer gets pretty dry and cakey. Never a problem.
PBW and it's ilk are my cleaner of choice. Put a few gallons of hot water & PBW in your keg, swirl and let soak until the gunk comes off. I do not fill all the way up to the top ring of crud so I usually have to do a little light scrubbing.
Disassemble and inspect the posts and tubes. Remove the lid and O-ring. I'll sometimes run a pipe cleaner through the dip tube. Soak everything in the cleaner for good measure.
For picnic taps, disassemble and clean the beer line. Regular taps can be cleaned every second or third (or fourth) keg. Fill a keg with cleaner and push it through with CO2.
With cold water.
If you cleaned your tap lines push some rinse water through them.
... with your favorite sanitizer. Soak the parts. Dump.
(Many sanitizers will retain their power if saved in a keg. I am thinking of making the switch to caustics for this reason.)
Whether you save sanitizer or dispose of it, reassemble the keg and charge with CO2 to seal.
Some people have a "clean tag" for kegs that have been through this process. I know a keg is clean if it is empty and charged.
The other two answers and their comments are great. For my system, though, kegs are something of a limiting factor and I rarely have an extra empty laying around. If there's not a batch in primary waiting for the old keg to kick, I'll have racked a batch to secondary and have it waiting there.
The ability to do this depends on the throughput of your brewery. (I have thirsty friends.) I would never attempt this if I had any of my containers sitting around for any length of time.