Well, don't do what I did, which was to spend $1500 on a mtb and then hardly ride it. Every time I start getting "into" it, I feel like I am ignoring the miles on my road bike.
I love the feeling of being in the woods, but I spend most of my time mountain biking screaming my head off and making sure I am not going to kill myself. I am sort of a weenie when it comes to risk, so I try to limit myself when it comes to the type of trails I go to. It takes so much "thinking" and concentrating, that it's a totally different experience. A few years ago I humiliated myself on a group mountain bike ride. Some people were riding 20 year old bikes, dressed in jeans. They were the ones who raced up the hills, while I pretty much walked 1/2 of the whole day. I know if I spent more time on my mountain bike, I would gain confidence, but I just feel the pull of my road bike.