I saw this trick shown on youtube with a number of dogs where their owners ask them to smile and they look like they are. Does anyone know how to teach this? I would love for my English Mastiff to raise his jowls and smile....

This may be easiest if you 'catch' him doing it naturally and then reward the behavior. Animals tend to repeat those actions that earn them something they want. You will need to be both quick and patient. Start by rewarding a smile with no teeth and then rewarding for a little looser smile, etc. If you use clicker training, it will go even faster. Best wishes!

(I don't subscribe to the idea of physically forcing a dog to teach a desired behavior - learning happens faster if the dog does it naturally, with a high rate of reinforcement initially)