I had a hard time going from competitive singles to doubles, but now I'm fairly used to it. Most of your brainpower has to go towards the consideration of that second pokemon your opponent will have out, and what second pokemon you'll choose. I actually took members of my singles team and then chose partners for them, and then cross examined them so that all four of my team could be proper partners with each other.
I was able to develop strategies that could exist no matter what my pokemon combination is, and I think that is the most important factor. You must always expect the unexpected, and be prepared to deal with your opponent using only two of your pokemon at any given time.
I use my Weavile and Dusknoir as examples for everything, but whatever. My singles team lead is Weavile, and after some careful consideration, I decided that I could keep his current moveset and blend him into a TR team. Dusknoir was a perfect partner, and between them they can handle almost every goodstuffs/frequently used pokemon.
My only troubled encounter was a Porygon-z. :/
Anyway, the point is that it is hard to get into, but once you're there, everything will be instinct. It's all about synergy, predictions, and split descisions.
