The way I train mine is with Beef blood. I start out laying a trail (blood mixed with water) and as the dog progresses the drops of blood get farther apart. With a treat at the end of blood trail, I use Cheese.
Some of it is natural instinct, certain animals excite them more than others and it can be a little different dog to dog.
I use a piece of hide to excite them, Pig, Fox whatever, tug of war, fetch whatever. They've already learned to follow a blood trail, making the leap from Beef blood to Hog or whatever you are tracking easy. They already have learned to be excited by certain scents from the hide, they make the association between play time and different blood scent easy.
I was told to avoid using game blood by an oldtimer, he said the scent overpowers the dog, which makes sense. He said a good dog can smell a single turd in a cesspool, their noses are that good.
Like App said a lot of it is being able to communicate with your dog, most want to please and if you can communicate what you want they will try to oblige.
Something else I figured out is most every dog has it's strong points and it's weak points, if you figure out what they are you can train to their strengths. I'm not a big fan of cookie cutter robot dogs.
I have three now and all have their talents. One is all nose and will follow a trail until he drops, if he gets away from you had better be prepared for a long walk. One is a flusher she will get into the brush and check out every nook and cranny. One is an earth dog, she lives to go down burrows and holes, loves it down there.
I had a dog that could follow a tiny drop of blood every 20-30 feet at a full run. That dog hated Fox and Jackals (Coyote) and would tear them to shreds if I let him. If he saw a Fox while driving he'd try to break through the windshield to get at it. He was just born that way, nothing I trained him to do. Hog blood would also send him wild, but not s bad as Fox or Yote blood. He was a bit conflicted with Deer, I taught him young not to run Deer and he was a bit unsure when following a Deer trail. He'd stop and hesitate and look back at me a lot to make sure he was doing the right thing.
Scent tracking is some different than blood tracking. It can take a long time before they stay on one trail and don't get crossed up or sidetracked by new trails and old trails crossing each other.