Began having mild RLS a year or so ago, but has become progressively more severe over the past six months.
Preventative vitamins such as magnesium and potassium have helped (I've also read iron, valerian root, and calcium can be beneficial, but can't vouch for them personally.)
However, if you are experiencing symptoms and need immediate relief I use controlled cramping, extreme stretching, and massage.
When I say 'controlled cramping', I mean contracting my feet in a way that causes a cramp, which is painful, but if you can flex your foot enough to moderate the severity of the cramp so it is tolerable for an extended period the relief after the cramping ends is great. I also do this for my calves.
'Extreme stretching' is sort of the opposite of my cramp technique. I'll stretch every muscle and tendon in my ankles and legs to whatever point I can stand. This is also painful at the time, but totally worth it if it means getting sleep.
Sometimes I'll also use a vibrating massager on the areas the RLS targets (usually outer ankles and up the side of my shin) before bed as a somewhat effective preventative measure.