Training your dog to touch their nose to your hand is...well, handy. It's also easy. Read on.
Steps one and two
Hide a bit of cheese or another dog treat under your thumb, with your hand otherwise flat. Put your hand about two inches from your dog's nose. Your dog will almost certainly investigate and touch her nose to your palm. As soon as her nose makes contact, say "yes!" and open your thumb to allow her to eat the treat.
After doing this ten times, you'll fake your dog out a bit. Put your hand out in the same way, but with no treat hiding under your thumb. When your dog makes contact this time, say "Yes!" and feed from your other hand. Do this ten more times.
Steps three and four
Now, present your hand in a standard hand target position, thumb to the side. Your dog should touch her nose to it, on faith. If she does, say "Yes!" and reward her with a treat. If she doesn't, head back to step one and do the whole thing over.
Repeat this ten times, and then make it a bit harder for the next step. Put your hand out a foot from your dog's nose. Do ten like that, then try a set of ten with your hand off to one side. Then another ten from across the room.
Adding the cue
When your dog is trotting across the room to touch your palm, it's time to add the cue.
Say "touch!" (or whatever, really...just pick a word that you want to use as a name for this behaviour) and then put your hand out to the side. Always say the word first, and then put your hand out second. Over time, your dog will learn to anticipate the hand coming out when they hear "touch!" and they'll head in your direction.
Always reinforce this behaviour with a treat, until the dog has been doing it successfully for six months. After that, you may reinforce every second or third one. You must continue to reinforce some of these forever, or your dog will simply stop doing it.