I don't play it often, mostly for farming and sometimes H Sindy, but it's basically deep freeze when it's up, frostbolt spam, CDs (mirror image/icy veins) when I have iceblock/coldsnap up (this is for H Sindy. I think Jay has a frost pve post somewhere down the page that might tell you how to better use your CDs on another boss). And Frostfire bolt when I get my brain freeze proc.
Some folks try to time the deep freeze for when fingers of frost procs, I think, but I don't bother. Same with ice lances, I think some folks try to sneak one in at the end of two frostbolts when FoF is up, but I think that's only a dps increase when you can pull it off each time (have low latency, etc).