If the buck doesn't collapse on the spot, watch it for as long as you can. Once you lose sight of it, pick a landmark that you can easily track to, to begin your search of the area - be it a large tree, a rock or a fence post.

Upon arrival, mark the spot and check the ground for hair or blood that can aid you in determining where the animal was hit. If you hit the buck in the liver, there will be brown hair and thick, dark blood left behind. If it was hit in the lungs or heart, there will be brown hair and a lighter colored blood there. If it was hit in the lungs, there may be little air bubbles in the blood also. If you hit it in the stomach, there will be white hair, watery blood and green-colored stomach contents. A stomach shot can usually also be determined by the strong odor of your arrow after it has passed through the deer.

Follow this trail to find your buck, placing markers within sight of each other, and keeping in mind that the wounded animal will often take the easiest path of flight. It won't usually go crashing through heavy brush or tight passages to get away because it will probably be panicking and because haphazardly fleeing expends too much energy. (Also be sure to walk quietly to one side of the trail so you don't disturb any evidence or alert the deer to your presence.)

If you lose the blood trail around your marker or don't see one, don't worry - sometimes lighter bullets and arrows (or a piece of an arrow) will stay inside the animal, causing severe internal bleeding, but preventing much from falling on the ground. Sometimes if an arrow passes through a deer cleanly, fat may plug the arrow holes in the deer keeping bleeding to a minimum. Just follow it to the place it was last seen and then walk in slow circles that spiral out from your marker, keeping an eye out for trace amounts of blood and hair or trampled brush that might clue you into the flight path.

Remember that fleeing bucks will always seek cover, so thoroughly scan the brush when walking your circles. (Another good place to look is near water - the edges of rivers, lakes and ponds provide the animal with a refreshing drink and a healthy amount of cover). Use caution when trailing a deer - if a deer is wounded it may be unable to run or walk, but may be completely alert and can be very dangerous when approached.

Remember to immediately tag your deer when you find it - its the law.

Note #1: It is infinitely easier to trail a wounded deer if you have 1 or 2 friends with you. This will allow one person to always stay at the point where the last blood was found to give the other person(s) a reference point as they look for more sign.

Note #2: Weather, visibility, and type of wounding play a major factor in when to trail a wounded deer. The organs hit and severity of the wounding would be your first indication of how long to wait before trailing a deer. Stomach and liver shots may require you to wait several hours before trailing a deer while heart and lung shots require minimal if any wait before trailing the deer. The weather will affect your decision as well because rain may necessitate the need for immediate trailing so the blood and other sign does not wash away. Visibility may play a factor in when to trail a deer as darkness can hamper any trailing activities. To avoid the factor of darkness, every hunting party should have one if not several lanterns (we use coleman dual-mantle gas lanterns as we find them to be superior to most other brands and models). Snow can play a part in the visibility factor as a heavy snow can cover up a trail and even an entire deer in a short amount of time. All of these factors must be taken into account when deciding how long to wait when trailing a deer.
 
(parts of this article excerpted from whitetaildeer.com)