Well, like someone said, it's like riding a bike. You never really forget the tecniques, maybe the finer points. My biggest problem is timing and lots of smaller things. Remembering to breathe, staying relaxed, keeping my center, etc.
I totally agree with chillzATL, I would just like to add that when trying to resume your training, your body's reaction might not be the same. I usually train regularly about 3-4 times a week then there was this time that I "rested" for about 1-2 months, and when I came back, I easily got tired and dizzy (and this was just after the preliminary exercises!). So my advice is...give your body time to adjust, just don't force yourself to train at the same level as you left, start slow and progressively increase your training level.