Q. How do I teach my dog to sit still enough and not move his head while I clip on the gentle leader?
A. Most dogs HATE the gentle leader, and it's not at all surprising. Would you want something foreign on your face? It's an uncomfortable training tool, and no dog enjoys wearing it. If you are looking to have your dog behave better on-leash, you should consider tossing out that gentle leader, and using a front hooking harness like the Sensible http://www.softouchconcepts.com/index.php/product-53/sense-ible-harness, or the Sensation http://www.softouchconcepts.com/index.php/product-53/sense-ation-harness harness. These harnesses will eliminate the pulling power of your dog, and put you in control in a positive, and gentle way. Any time your dog pulls, he is redirected until he is facing you. You can practically walk your dog with your pinky.
I dislike the gentle leader because it can cause neck injuries in an avid puller/lunger. You also can't ever hook a long-lead to the gentle leader and allow your dog to run around because it would break his neck. Another thing I dislike about it, is it discourages sniffing the ground during walks. When your dog attempts to sniff, and the leash is short, his nose is redirected upwards. When you trip on the leash, the head is jerked around and the nose is directed upwards. Sniffing during walks is extremely important. Sniffing = mental stimulation, which will tire your dog out more during your walks. The more your dog lags, or forges, the less he can sniff the ground, and the more frustrated he becomes.
If you're dead set on using the head halti.. you should be using treats to hold his attention. Place the head halti on the floor, reward him for sniffing it, pick it up, treat him, put it near his face, treat him, lure his nose through the loop, lots of treats, take the head halti off, more treats, lure his nose through again, more treats. Take baby steps going forwards AND backwards so the "game" of getting the halti on isn't always getting more difficult.