a) Ensure that you know where you are going, and that you are able to access it. See the article on rights of way  for more
b) Use the ABCD check  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.