A question often asked, for obvious reasons.
I guess there are probably a number of methods, but mine is to place a thin strip of white card, with a slight kink in it, to lift the near edge up a bit and to face forward, under the front of the box. At the same time I have a couple of shallow blocks (about 4mm) placed under the front edge of the box.
Works for me, but maybe not for everyone.
Edit: I have a recollection that some others may have used a bit of kitchen foil in some way.