I didn't want to use a clicker while teaching him to fetch, because I didn't really want it to feel like a 'training session' per day. He knows when that clicker comes out, it's 'business' time and I really wanted to teach him to have FUN with it. Not be... focused necessarily.
He used to fetch a ball maybe once, and then bring it halfway back (if that) and be interested in something else. He was definitely not a big fan of fetch. He would play inside sometimes down the hallway, but hardly at all outside, especially not in strange / new places.
Now, HE LOVES it, and gets super excited when the Chuck-It comes out. He will sprint up hill at full speed to fetch the ball or frisbee and bring it back all the way, and bark at me begging for more! I take him up to a local football field and he plays there now too.
I used treats in the beginning (and actually still do every now and then, just not after EVERY throw, but just to reinforce) and put him on a long line he can drag. Then I'd get REALLY excited! Like, I'd really act like an idiot. I'd throw the ball/toy/etc and run for it and show him how to do it and act all excited. He started going to get it himself and if he didn't bring it back all the way, I'd give a slight tug on the leash as a reminder. If he did not grab the toy on the way back, I'd kind of 'make' him. Like I didn't keep going to pick it up from wherever he dropped it, I just kept reminding him to bring it all the way. I kept encouraging him to go get the toy, and it took a couple of times for him to leave me (since I had the food) but then he figured out what I wanted. I started with short distances.
Since I didn't really want it to be a 'trick' or a 'command' but more of a game, I started using playing as the reinforcer occasionally too. So, one time he'd return and get a treat, the next time, I'd let him tug on the frisbee or chew on it for a few seconds, etc.
I also tried many different types of toys. He really likes the rubber ChuckIt mini balls as well as the Kong Squeakers. It's been a great added form of exercise when I'm too lazy to leave my yard and to to the park