Can anyone give me advice on how to train a dog how to stay with you when you go trail riding? I know there are lots of people who do this, but I don't know how they do it. LOL I'm assuming it probably mostly has to do with the training of the dog?

Your right. You first need to have a good handle on your dog. But I know with my friends field trial dogs, they wear harness and a long rope style lead is used and attatched to the saddle. But before you do this you need to make sure that you practice in a round pen or arena.. But I know with our dog, we started him our as a puppy and he went everywhere with us and was always around the horses, so he just naturally stayed with us when we are on the trails. You could always speak with a dog trainer and see what they recommend.

Good Luck

I'm running across this a bit late, but I thought I would throw in my 2 cents worth. With a horse you need to establish that you are leader of the herd, with a dog you need to be Alpha in the pack. Dogs are still essentially pack animals and most of the time think of us as pack leaders. If you can think with that mindset (or the mindset of any animal in it's natural state) you will have a better chance of getting it to do what you want.

Like P8nt, the dogs that went with us, stayed with us because they wanted to. That is not to say they didn't occasionally follow a good scent or chase a rabbit or squirrel (dogs gotta have fun too!). But the most important thing was they understood when you gave the call or whistle for them to come back, that was the end word, no discussion, no arguement.

I have always taught all my animals to obey commands instantly. In the case of a friend's dog, it saved her life! (Queensland Healer in a rural area, gave the down command while crossing the road and a car drove over her with no injuries.) This requires a lot of training and complete trust by the animal. But after seeing the importance of complete obedience I began to do the same with all my animals -- well okay, nothing a cat does is instant LOL -- my cats DO obey simple commands, they just complain and grumble about it the whole time. >^,,^<

If you don't have obedience off lead and off trail you won't have it when they are riding with you.

The other thing we taught all the dogs (most were med/large australian shepherds or smaller breeds) was to ride the horse with us without fear or fuss. This is important in case the dog injures a paw or something and needs to be carried back home. Again, do the training ahead of time. Finally, always carry a little food and water and first aid stuff... just in case!

If your dog is comfortable around horses, you're off to a good start. Dogs who are overly cautious of horses dont usually tend to want to follow.
If your dog is cautious and wary around horses, work first around the barn by leading yor horse around and encouraging your dog to follow right along. Treats come in handy.
Really reiterate the "come" command. I always use treats. Teach your dog (off leash) to stop whatever he's doing by calling his name. Soon as he looks at you, throw him a treat. That was he associates his name with a treat. (if your dog is food oriented, I have labs so its easy, heh heh!) It's so annoying when you call a dog and he completely ignores you, especially when you're powerless on a horse.
I have a 6 month old lab/pit puppy who I started riding out in our jump feild and had her start to follow me around.
I would take her & her brother(as soon as about 3 months) and my older dog out for walks where we trail ride, off the leash, and whenever she got too far ahead, I'd call her back, throw her a treat and praise her for coming back. That way your dog already knows the drill when you hit the saddle. If your dog tends to just ignore you and run around and pay no regards to you, ignore your temptation to put him on a leash. Watch him like a hawk and if you feel he's about to take off, call him back.The more you catch him having the urge to run the more he'll learn he's not allowing to. Be patient and call him until he comes, and reward with a treat. Leashes dont teach them to stay with you, I dont think, only forces them to so they never learn to behave off the leash.
It also helps to take other dogs that know how to trail ride with you so your dog can follow and learn that its a fun thing dogs do.
Also, your first few rides dont go too long. If you go too long and your dog tires out, it spoils the fun for him. So go a few times on short rides until you feel like hes enjoying himself and gradually build the distance. You want to make it a fun, positive experience so your dog wants to come.
One more thing, I had a dog that was super hyper and really enjoyed running way too far ahead of us. One time we took her up to a friend's in the mountains and she high-tailed it too far away and wouldnt come back. I about had a panic attack, thinking we lost her for good. A little before we got back to the trailer, there she was. Looking quite unhappy w/ herself. After that she always stayed close in fear of getting herself lost.. Sometimes they need a scare like that to teach themselves to stay close with you.
I hope that helps! I've had a bunch of experience w/ this, so if you have q's feel free to ask!