It might surprise you to see this next on the list. Most dog-training courses have you teach "down" before "shake". However, as most foxes are extremely adroit with their paws, shake is a very easy trick for them to learn--they'll pick it up much more quickly than "Down", and this will help to build both your confidence and your pet's.
Prerequisites: Your fox must be conditioned to the clicker, and know the command "Sit".
To teach your fox to "shake", have it sit in front of you and hold the treat in your closed hand on the ground, palm up. Your fox will probably sniff and lick at your hand at first, and may even nom you gently, but don't release the treat. Generally, he will very quickly shift to pawing at your hand to get it open--the instant he does this, click the clicker and release the treat, praising him enthusiastically.
Most foxes catch on to this very quickly, and will start tapping your hand right off the bat. Once he does this, lift your hand a little bit off the ground when you offer the treat, and say "Shake" first. Again, this generally takes very little time to master.
The next part is a bit more difficult. Start offering your right hand (Still with a treat in it) and telling him to "shake" but rewarding him with a treat from your left hand. This will take a little longer for him to understand (I hit hand A, but hand B rewards me). Once he does, though, you don't need to be holding a treat in the offered hand--just offer a flattened palm for him to paw at.
From there, just gradually build up what he has to do to get the treat. At first, just tapping the hand is sufficient. Next make him hold his paw there for a moment. After that's been mastered, he has to let you curl your fingers around said paw. Eventually, you get to the point where he has to let you hold his paw and shake it gently.