First of all make sure to clean a hell out of screen glass - the more cleaner the glass, the better adhesion.
Now just take few stripes of adhesive tape, and make an "T" shape out of them, in a way shown on picture. If you have narrow adhesive tape, you should probably need a few "T" shaped stripes that you'll connect later on, so you can have bigger surface, or just go with one stripe of some wider tape - use whatever you have lying around.
Now you just affix those stripes onto the glass right above the Home button, you firmly grasp the metal sides of iPhone with one hand, and start to pull tape with the other. The smaller (lower) the angle of pulling in relation to the glass, the better the grip.
Glass won't just pop out right after first pull as it would by using suction cup, but after a few metered pulls it'll pop right out.
There you have it - ghetto suction cup.