My biggest plan for a scene is simply what I do at night...which, I have a feeling, you don't do too much of. I go to sleep. I read a couple stories before I go to bed, get myself in that certain frame of mind, then just doze off. I usually have a dream about what I would like to do, and while that doesn't have to be exactly what happens in the scene obviously, it's a good starting point. I'ts a lot easier to realize what you want when you aren't trying to think of it. Of course, during the scene, things are likely to change, due to mood, time, feelings, wants, what have you...but thats always going to happen.