a) Ensure that you know where you are going, and that you are able to access it. See the article on rights of way [1] for more
b) Use the ABCD check [2] to make sure your bike is good to go. Your tyres should be a lot softer than on the road - you should be able to squeeze the sides of the tyre a little bit
c) Your head and shoulder position is key when riding off road - look where you want to go - you will go there (so don't look at that tree you want to avoid!) and keep your chin up - it changes your whole body posture
d) Be prepared to stand up, keeping your pedals level and your elbows and knees slightly bent and soft - this is your suspension, and comes a whole lot cheaper than some of the bikes out there.