Forgive me for being a little late on this post, but I'm new here as well. Since I do mostly critter's myself, I got a couple of tips for ya...
The first is simple....aim for the eyes. This really holds true for any kind of portrait photography, but very simply the sharpest point of focus should always be the eyes. If the eyes and face of whatever your shooting is out of focus, forget it...you'll end up with a bad shot every time.
Secondly...and this is sort of a general rule of thumb but, try to keep things at eye level with whatever you're shooting. I'm sure you've seen tons of pics that people take of their dogs/cats, etc and more times then not, even if the focus is perfect, the pic usually looks like a "snap shot". Why? Because the person is standing above, looking down at their pet. There are many places where this rule can be broken if you know how, but for a begginer, I say get down on the floor, ground, whatever and shoot at eye level.
Third...always apply the basic rules of photography where ever you can. Good composition, the rule of thirds, sharp focus, etc., all go a long way towards any great picture! Frame in tight on your subject and keep your backgrounds simple and uncluttered wherever possible. I personally try to keep a very limited DOF (Depth Of Field) when I do critter portraits, but thats just my personal taist. Since you said you were a newbie, if you don't know any of these terms, head to your local library and get out a couple of books on basic/introductory photography.
Fourth...know your camera! I can't really state this enough...working with animals is something of a challenge at times. They very much have minds of their own and aren't always willing to cooperate with a photographer for more then a moment in most cases. If you're standing there fiddling with your camera settings or trying to remember where something in the camera's menu system is, then chances are, you've already missed the shot. This is just my own personal experience here and I'm certain there are those who would debate this, but I usually shoot with most of my camera settings on automatic...or at the very least, in aperature priority mode (again because of my thing about DOF). Wildlife photography is NOT studio photography...they take two very different techniques. If you're worrying about the lighting being slightly off, or your sitting there trying to calculate the proper f/stop to shutter speed ratio, as before, chances are you've already missed your shot. Know your gear BEFORE you head out into the field. If you're an absolute control freak, chances are wildlife photography isn't for you.
Fifth...and again this is just me, but I have to credit this as one of my biggest key's to success with critter pics, but shoot -LOTS- of pictures!!!! Very simply, even if a person knows absolutly nothing about photography, if they shoot enough pics, chances are that sooner or later, they're going to get a great shot! LOL!!! If you're not shooting digital, I HIGHLY recommend you consider it for shooting wildlife as the more pics you can shoot without having to stop and reload film, the better your chances of getting some great pics. Except for special situations, generally speaking with film, you can get 32-36 exposures before you have to reload. On my Sony H1 with a 1 gig memory stick, I can shoot almost 400 pics before I have to change memory or download. I've actually stood in one place at the zoo for example and just shot off 30 or 40 pics (or more) of the same animal trying to get one or two good shots. Think of it this way...you have the critter all framed up...the lighting is great, the animal's expression is priceless and just as you press the shutter button, WHAM, the animal turns it's head to look at something else. Your picture turns out to be nothing more then a blurred head pivoting on the animals body...probably ok for surealizm, but not usually a good thing for wildlife photography! LOL! I'm not saying that you need a camera that shoots 8 frames per second or anything, but the more you shoot, the better your odds are of getting a great shot instead of simply a good one...and the better your skills as a phographer will get as well.
Sixth (not getting too much for ya I hope! LOL!)...KNOW YOUR SUBJECT! Especially if you're planning on shooting in the wild, you need to know a little about the critter(s) your planning on shooting. If you're looking to shoot foxes for example, you need to know where to look, you need to know that they are primarily nocturnal, knowing their eating/feeding habits etc., will all play a role in creating great images. Did you know that hot dog buns make great "bait" for racoon's, but spoiled lunch meat tends to attract skunks? Definatly things worth reading up on
A couple of quicky's...in your original post, you stated you were using something like a 500 or 600mm zoom. Don't forget to adjust for the problems associated with using such a long lens. Even the slightest camera shake can ruin a pic at those focal lengths! Typically, you'll also loose some light so watch your shutter speeds as well. As was mentioned before, a great tripod is essencial. This may sound dumb, but another thing I consider absolutly essential for wildlife photography (either in controlled situations or in the wild) is a really comfortable pair of shoes! Aside from camera gear, remember to dress apropriately...if your hiking thru jungle-like forest, flip-flops and cut-off's are probably going to give you problems! LOL!!! For digital, extra batteries and an extra memory card or two are always welcome items.
Lastly...and this is probably the biggest of all...patience. Shooting wildlife pics is almost as much about luck as it is skill. Having the right critter at the right place at the right time maybe doing somethign photo worthy (other then just standing there)...even in controlled circumstances like the zoo it don't happen much. If you're actually shooting in "the wild", it's not uncommon to have to sit in a "hide" for several hours (days?) on end and still walk away without a single shot, let alone a great one.
I could go on and on about this, but hopefully that will give you some stuff to work with and I wish you luck on your up coming expedition!
Bright Blessings & Gentle Breezes to you,