I use napisan as it's cheap to buy in bulk, and means I can go a little crazy with it.
1. Clean as well as possible just using water
2. Fill with water and add about 4 heaped tablespoons of napisan
3. Seal keg, shake well, soak for 15 minutes, invert, soak for 15 more
4. Flush through lines under pressure
5. Rinse well again with water (I know, this seems silly as I've just sanitised, but I've not had any problems yet)
I usually do this just before filling with beer, so nothing nasty has a chance to take hold before the alcoholic beer goes in and makes it inhospitable to bugs. :)
There are probably better methods, but this works for me.
I use caustic - just rinse the keg with water after emptying. Spoon a bit into the keg and fill with water. I leave that for a bit - depending on when I'm going to refill it and how lazy (I mean busy) I am. Then rinse again and use iodophore. I push this out with CO2 when I am about to fill it. Occasionally I will take the posts off and clean them separately prior to sanitising.
Well sounds like I'm a bit less thorough than others here!
I use good ole dishwashing liquid to do the washout after use. Swirling, shaking, draining a few times until it smells clean rather than like hop residue! I also put a bit of gas through it to push any gunk out of the beer diptube.
I then give them a rinse and a quick swish of iodophor before storing. I repeat the iodophor just prior to using.
I've only gone through about 7 kegs but I've had no problems.
I clean my taps but so far haven't dared to fiddle with keg bits. Might have to in a few kegs time.
I use nappysan too. Wipe away the tears, then rinse the keg out when it blows dry. Half fill with water from the hot tap, half a lid of no-name nappysan, shaken not stirrred.
When filling I pour out the nappysan, rinse well with cold water, throw in some iodophor solution, shake, sit, upturn, sit, empty and rinse with boiling water. Connect a line in and alternately a line out, blowing some of the boiling water out of each (boiling water brings up enough pressure to do that without gas). Then fill, chill, gas, shake and drink and start slurring. In that order.