There is a video out there on the net somewhere(tried to find it) that shows proper hand position for shifting. It helped me a whole lot, and one thing it taught me was that your hand should never be wrapped around the shifter.
When I first got my car I would try to force it onto every gear and missed third quite a bit as I would push it over to fifth. It's really about finesse and I haven't had a bad shift under heavy acceleration in a long time.
Does anyone know what video I'm speaking of? I could have swore it was posted on the forums a while back.