Consistent contact and a lot of leg. It actually happens naturally as soon as they start pushing from behind. You don't want to pull them into a frame. Not only is that impolite to the horse but it accomplishes nothing but teaching them only to bend their neck so you will leave them alone.
Hi! Im guessing by outline you mean frame and if that's what you mean then I like to do a few things in my warmup to help. First of all, right when I get on, I bend my horse hard right and hard left to get him to soften and then do that in each gait during our warmup. I also like to do lots of small circles, cerpentines, and transitions. Whenever I feel my horse lean a certain way I bend him so he moves away from that leaning side. Hope that helps and makes sense! 😊