When I found Kobi at the shelter, he was one of the few puppies to have come to the shelter already knowing how to sit. Once we got him home teaching him basic commands such as sit and down were a breeze thanks to whatever training he got from his previous home. However from day one we needed to turn his sit into a default "please" command, which meant making him sit for everything! 8 months later Kobi is now able to sit almost immediately on command and give eye contact (sometimes brief), the second I ask him to sit. He has also turned sitting and looking at me into a default command. If I stand at the door to go outside, after a few seconds of the door not getting opened he sits and looks up saying, "please can you open the door now?" without needed any command. We use sit for everything, and I mean everything. He must sit before getting his food, before going outside and even when out on a walk we get him to sit before he crosses the street. Sit has become one of the only commands I can give Kobi when he does get excited, but it is because of the large amount of work we do with it.
Everyday with produced an opportunity for training. I choose to make sit his "please" behaviour, but any trick can be used, or multiple tricks can be used. The point is to make your dog say please before he gets what he wants. We even use this concept of "please" when we're out playing. When I do play fetch with Kobi he must do a trick before I throw the ball. By getting them to say please for the ball being thrown is a great way to work on newly trained behaviors, it's great to have a default behaviour, but it is also very helpful to have a high motivational item such as getting the ball thrown to get them to do other things, sometimes i'll even do a round of commands before throwing a ball. "sit, down, sit, paw" then throw. You don't need to always have treats with you when working on default behaviors, usually the continuation or progression of what you were doing is enough of a reward. For example when walking, getting to walk again after sitting is the reward, or getting the door opened to go outside is the reward or finally getting that ball thrown.
When I started training Kobi's default sit I was more focused on getting him to sit, now I am working on getting him to look back towards me before he gets what he wants so that I can work on his focus and attention towards me. Sometimes he gets frustrated thinking "well I sat, why aren't we walking again" but he works it out eventually sitting again and looking at me. If he seems to be really struggling to think through it I will cue him to look and sit to remind him what he's supposed to be doing, usually by the next time we come to getting him to sit and look he figures it out on his own.