Practice. It's literally all you can do.
Personally i'm a halfway decent dancer but I rarely use it, I usually just bob my head and move my body along with the music it's piss easy.
Good routines can come from being able to dance well though. Me and my buddy have one where he pushes my chest down and I follow his hand without touching and then back up; you've probably seen it on a music video or something it looks similar to the popular one used from the song Low by Flo Rida. This ALWAYS gets a positive reaction from nearby chicks who then come watch then we break into some other cool moves then randomly grab a girl each and have a "fake dance off" parading our girls. This works about 80% of the time, obviously it doesn't work if even one of the girls disagrees to dance but they usually do (if they say no while laughing with you you're gold, grab em anyway) if only one joins we allow the other to do his thing.
You don't have to be a "good" dancer to have good dance game though. You can pull girls doing this
http://www.youtube.com/watch?v=96eSrFlUVh0... I've done it...