Don't just be a dummy master though, I have a friend who sat on a dummy and could pull really decent numbers on his rogue.. As soon as he steps into a raid and has to deal with movement and mechanics his dps turned into shite.. Learning the basic playstyle of your spec is one thing, learning to do good dps is another. You need to apply more of a "how to do good dps on this fight" approach to every fight, rather than "how do I learn my rotation".
Proving grounds is good training, puts you under pressure and forces you to adapt your own strategy to maximising dps in the situation. Otherwise your best bet is to do it in raids, even LFR.