A sit does not really mean much when the dogs is not asked to apply it for his/her safety, patience, and politeness in real life situations. Once your dog knows to sit, ask him to sit and wait by teaching him a release word such as "Free" or "Okay" or "Break."
Once your dog knows to sit and patiently awaits for his/her release, start applying it and use it in a way to ask him for his patience; as if he/she were to say "Please may I?"
In our training classes, we teach the dogs to sit before exiting:
1. Their crates
2. Front and back doors leading to outdoors
3. Coming out of the car
4. Leaving all public places, such as PetSmart, PetCo, Univesity of Doglando, Groom Grub and Belly Rub.
In addition to asking them to sit before every door, ask your dog to sit and wait every time you stop on a walk. For example, while you talk to your neighbour or mail carrier.
Your dog should not break the "sit" until the release. In the training phase, be sure to reinforce your dog waiting by rewarding with a treat as long as he/she maintains his/her position, but once your dog gets good at this, let the reward be at the end.
In our B.S. in Dogology and M.S. in Dogology classes the reward is presented at the end. This makes for an anticipating dog, a dog who is so tuned in to you, and only wants to hear the release word.... he/she does not really care about anything going on around him/her.