Use tape to clamp and only glue where you have to.
End miters are easy but what do you do when you have to glue a long edge miter, where the glue is slippery and causes the pieces to slide out of position?
The simplest way is to put painter’s tape across the joint before it is glued, fold the joint open, apply glue, fold the joint closed and use tape to keep it that way until the glue has cured. Tape is cheap, goes on quickly, and prevents the pieces from sliding around.
For stain-grade work you can avoid a visible glue line by gluing the heel of the joint only. If you don’t use too much of it, it will spread when you fold the pieces together but stop short of the corner and won’t squeeze out. Lightly sand the corner and you should end up with a joint that’s completely invisible.