What breed of dog is she? Does she have long hair around her mouth? If so, you can have a groomer (or you can do it yourself if you have clippers) give the hair around her mouth a close shave, so to speak, to remove the hair that the food and dirt cling to. Eliminating the hair should eliminate the problem. I tend to agree with you that it's a combination of saliva, dirt and food build-up, and it's pretty common in dogs with "beards".
Ghilly
Keeping the hair trimmed is a good idea. You could try rubbing cornstarch in the hair and then try combing it out it not too matted.