There are so many reasons why it's a bad idea for a dog to jump at the door, especially a glass one. From the large dog who accidentally smashes the glass to a small dog leaving paw prints, keeping 'four on the floor' and not jumping up is the best idea.
Sorting this problem requires a little lateral thinking. First, you must figure out why the dog jumps at the door. For example, is it his way of asking to go out to the toilet or does the dog get super-excited when the doorbell rings? Identify the role played by jumping and you can teach an alternative behavior that is both safe and acceptable.
As with many training issues, prevent the dog doing the unwanted jumping while you're retraining. This can be as simple as putting the dog in another room when visitors are due or leaving the door to the yard open when you're not training.
Jumping up is an action the dog teaches himself. He learns that jumping up makes the door open or visitors appear. This means the behavior is ingrained, which makes it harder to break. This is why it's important to reduce the opportunity for the jumping, while you retrain.
Successful trainers don't just tell the dog what not to do, they put an alternative action in its place. That way the dog knows what is expected in order to make the door open or the visitor enter.
Train little and often, say twice a day for 10 - 15 minutes at a time. Always used reward-based methods and keep the sessions fun. Last but not least, end on a positive note with a command the dog is able to do.
All you need are basics, such as tasty treats. In addition, you'll need props such as a mat (for the dog to lie on instead of going to the door) or a bell (to ring instead of jumping at the door).
Be aware that punishing the dog for jumping at the door is ineffective and damages the bond between you. Instead, teach the dog the appropriate response so he knows what's expected from him.