>> Remembering simple things like approaching slowly, keeping your silhouette small and making no noise are great ways of getting closer.
Great advice. Remember too that wildlife often makes better use of their senses than we do. Approaching slowly and silently will do you little good if they can smell you coming, so wind direction is something else to pay attention to.
Shooting from a blind can help but remember that birds aren't stupid. Most of the time it's recommended to set up a blind a few days in advance to give the birds time to get used to it... this does work, but if the birds are there when you enter the blind, they know darned well you're inside even if you stay there all day. On the other hand, most birds don't count very well... so if you can't enter the blind while it's still dark, just have someone walk to the blind with you and then have them leave alone, most of the time birds will assume the blind is empty.
If you don't have a blind, or need to move from place to place, camouflaged clothing helps a great deal. I've found that a lightweight 3D leaf suit ($50-$100) is about the cheapest and most effective way to cover yourself. Most are comprised of mesh camo fabric with leaves sewn on... this breaks up your silhouette more effectively than plain camo, but is lighter and cooler than a ghilli suit. I also like to wear a turkey vest (rather than a photo vest) to hold my gear because most of them have drop down pads in the back to sit or kneel on. Top the outfit off with camo gloves and a facemask, and you can blend in very nicely:
Most wildlife is highly sensitive to movement so even if you are well-camouflaged, approach carefully. If you are stalking one individual, try to keep a tree or bush between you and the bird as you get closer. It sometimes helps to move when his head is turned but the angle of vision varies by bird so do some research. Keep your eyes moving... you have to not only watch your subject but also other wildlife that might get spooked and start a chain reaction of mass retreat. You also have to keep looking where you are stepping so as to avoid noise. Going out after a rain helps, leaves won't crunch as much. Wind can help mask your noise too, as well as creating leaf motion that can mask your movement, so choosing a breezy day or waiting for little gusts to disturb the leaves can help a great deal.
Pay attention to light... it seems obvious, but if you approach behind a tree or bush, favor the shadow side when you peek out to take your photo.
Depending on the area of the country or time of year, snakes might be a concern. Being quiet while stalking a subject is one thing, but most of the time you'll probably want to make some noise while moving from place to place... most snakes are shy creatures and will retreat if they hear you coming.
Finally, don't be too single-minded in your focus. Watch for other subjects like flowers and insects that might be right at your feet. Scan the skies too.