The first (and arguably the easiest) trick to teach a fox or most any other four-legged creature is to sit. You can start teaching this one very early on. Gizmo had a good handle on it by the time he was ten weeks old. It makes for a great "trick preschool", as it's easy to learn and gets your fox used to the whole training routine with very little stress or confusion on their part.
Another good reason to start with "Sit" is that it's something of a gateway trick. A lot of the tricks I'll be teaching later on involve starting with a sitting fox; it's much easier and more effective to have them sit on command than to try and manhandle them into position.
Prerequisites: For this trick, I assume that your fox has already been conditioned to a training clicker. "Eyes On Me" is also useful, though not necessary.
For A Kit:
This is easiest if your kit is in a pretty mellow mood--don't try to teach sit right in the middle of playtime. You want him to be alert, but not bouncing off the walls. Get something your fox really likes (ham, little bits of hardboiled egg, and cat treats all work well.)
Sit on the floor with the kit standing in front of you. Hold the treat so that he can see it and smell it. If he noms on your fingers, do NOT release the treat, or you'll be rewarding him for biting at you, which is not the lesson you want to teach.
Slowly move the treat over his head, right between his eyes and back towards his ears, and say "Sit" (Only say it once). To continue watching the treat move, the kit will sit down to keep it in his field of vision. The second his rump hits the ground, click the clicker and give him his treat, praising lavishly.
Repeat this a few times a day, and he'll have caught on in no time.
Troubleshooting: If your kit turns around to watch the treat instead of sitting down, you're moving it too quickly.
For An Adult Fox:
Get something your fox really likes (ham, little bits of hardboiled egg, and cat treats all work well.) Let him see that you have the treat, then hold it up over his head, out of reach. Calmly say "Sit."
The fox will probably respond by jumping for the treat, and perhaps screaming hysterically. Don't give it to them. Continue holding the treat out of reach, and repeat the command "Sit" every now and then. Eventually, your fox will sit, if only to give you an exasperated look. The instant it's rump hits the ground, click your clicker and toss it the treat, enthusing "Good sit! Good fox!".
Repeat this process a few times in a row. I recommend 3-to-5 treats, 3-to-5 times a day. After a week or so, your fox should have definitely caught on that sitting down is what's getting them the treat. Once this happens, you can work on "polishing" the trick.
Polishing The Trick:
You can increase the amount of time that your fox has to stay sitting. Tell your fox to sit, and do a brief count in your head before clicking and rewarding. 1-2-treat. Later on, up it to 1-2-3-treat. So long as you increase the amount of time your fox has to stay sitting slowly enough, they should pick up on this in no time.
You can also tighten up their response time by making it so they have to respond faster to get the treat. Give the command, and mentally count to five slowly. If your fox doesn't sit within that count, don't give them the treat. (Note: If they never sit that quickly, you're starting with too short of a count. You might have to start with a seven or even a ten count, depending on your fox and how quickly you count)
Once your fox is consistantly sitting within the count you've specified, subtract a number from it--in this case, make it so the fox has to respond within the count of four instead of the count of five. As always, make sure these changes happen slowly and gradually, and your fox will be responding to you in the blink of an eye.
Oh, it's also best to focus on ONE area of polishing at a time. If one day you're working on making them sit longer, and the next you're trying to make them sit faster, your fox may become confused and frustrated, as they're not sure what you want. Focus on one or the other first, and once your fox has mastered either sitting for a while or sitting quickly, then move on to the other one and focus on it.
Happy training! If you have any questions, feel free to ask in the comments section.