[sapilla]

The most important thing is to have the tip of your tongue (almost) touching your teeth and then gradually increasing the amount of tongue-to-roof-of-mouth surface, beginning it the front - your current limit is where it blocks nasal breathing. This stimulates horizontal (forwards) bone development vs. vertical (downwards) development.

A "drill" I can recommend is writing out words a defined number of times (pick a number you like between 10 and 100), as you write also pronounce them without letting your tongue lose contact with the roof of your mouth. This may feel very unnatural at first. If you don't like the result just recycle the paper.

There's also the habit of sucking or pulling in your tongue - don't do that. I think it causes jaw clenching. If you want a fast way to get rid of that, invent metaphors for it and then watch yourself do it automatically in social interactions.

Also check out ortothropics, it's the emerging science of facial development through oral posture. They have a large collection of instructional material on YouTube.