Heya,
1. Get close to your subject. Long focal length simply doesn't make up for being closer. Everyone thinks they want a 1000mm F2.8 lens (obviously doesn't exist) until they try to shoot it through environmental conditions or over water and wonder why it doesn't look nearly as detailed, crisp, etc, as someone with a 500 F4 at 20 feet does. A really big tip for wildlife & birds is simply to get closer. Study your subjects. Get to know where they will be to do their thing. Be there first. A blind or hide is a great way to make yourself not obvious. Just walking a trail and hoping to get a shot might work sometimes, but not the great shots more often. Also when you go out, choosing time and location helps you choose how to control where the light will be coming from based on what you want to see. So scout, get to know your subjects, get their early, stay there, be patient, prepare, and get your shots. Easily the best thing you could do for getting "good" shots that is totally independent of good gear, this applies whether you're sitting in a blind near a feeder with any quality dSLR and a short but great 200mm lens, or an awesome 600mm lens. Still applies.
2. Get low. Perspective is a big changer in the "look" of a shot. Some shots are just typical from one angle, but amazing from another angle. I find that getting low is more often that angle you're going to get the "wow" shots from. Why? Well, it's about making the background as distant as possible and getting a perspective of the animal/bird that you normally don't see every day from your standing height. If you think about it, a small animal/bird or anything really from a standing height looking down a little will have the ground as the background all the time and it's not that far away, so it doesn't isolate the subject as well, regardless of having an awesome super telephoto with F2.8 or F4 even for depth of field purposes, even at close range. But if you drop down, and the ground is not behind the creature from that perspective, and the background is very distant, your subject will have great isolation, and you'll see a perspective less commonly seen from standing height. It pops. Obviously this applies to things on the ground. For things in the sky, or up in a tree, this is not something to worry about. But I will stress, no matter where or what it is, try different perspectives and angles, it changes things a lot.
3. Super bright hard-shadow-casting light is not the best light for most things, but it can be used. I like overcast days the best, more even light, less hard shadows. Requires more ISO or a slower shutter, but I'm ok with that. Be mindful of the sun's position. It will tell you what will be in shadow, while your eyes see it fine, the camera will not. Light really depends on weather, time of day, and using it to get the look you want is all on you. When I shoot song birds, I ideally want them out of direct sunlight completely, under canopy, in shade. It's dark really, and requires high ISO, large aperture, or slow shutter speed--all of which I'm fine with. When I'm at the coast, I have no choice, I'm in super bright reflecting sun with hard shadows, so I try to keep the sun on my back so I'm not getting underexposed subjects that are strongly backlit.
4. Expose up. Over expose your subjects a little. It really helps with detail and getting more pop out of your animals/birds. Nothing too crazy, but I go as far as 1.5 stops over exposed on birds, especially if using higher ISO. This may mean using more ISO. Be ok with that! Keep in mind that if handholding or shooting something in motion, a fast shutter is very important, so it may mean a lot more ISO than you really thought was ok. But be ok with it!
Very best,