Swinging for Kings:
Cast across and slightly downstream - This will give plenty of time to mend and get the fly down for that long, slow swing. I like to cast right across, throw a upstream mend and with the rod tip raised up, slowly back the fly down til it starts to swing (gives the fly that plugging appearance, backing down in front of them).
Throw a big mend upstream - Kings like the fly slow, a big upstream mend will slow the fly down and help get the fly in their face.
Don't set to early - The classic King take feels like, Headshake....Headshake..............Pull... Wait for the pull before you set the hook. If the King immediately starts screaming line off your reel, you don't need to wait, it's game on.....
Put the Boots to 'em - Once you set the hook and the game has begun, after that first run, fight them hard. If your not working, they are resting. The longer the fight, the more chance they will get off.