It is rare to have control of your lighting in the wild, so learn how to use what you have got!
Skills developed with other types of photography are directly applicable for wildlife and nature!
Keep the background in the foreground of your mind! :)
When it is dark, underexpose. When it is bright, overexpose!
Keeping your camera level, as opposed to shooting up or down, will get you better images (in general).
Sometimes you need to bump you ISO higher than you are comfprtable with. Do it. You can fix noise, you can't fix an image with unwanted motion blur!
The eyes are key to a good portrait.