I personally enjoy long strokes. Start off with short strokes introduction, to get it in, and making sure everything is all lubed up appropriately with juices. I will usually start doing long slow full long strokes. This would lead into my personal favorite of pullout out almost all the way slowly, and pushing all the way back in hard (rinse and repeat). Then from there i'll gage what the girl is wanting or liking. Usually they will say something like. "**** me as hard as u can", or "go slow, go slow, go slow" Just need to change it up so the body doesn't become too accustomed to the sensations. So obviously throwing in short strokes is good too. This is also a good reason to switch positions. I could go further into the explanation, however i kinda forgot my point, so i'll just say this.
The girls i've been with always enjoy the long deep strokes, slow, and/or fast depending on the girl. But short strokes are good too...