AnjelicaMirja: There is a difference between nipple clamps and a tit press.
Smaller breasted women should be seated, leaning slightly forward because it helps extend the breasts so they can be clamped by a press. Nipple clamps can be used easily in any position.
Nipple clamps clamp the nipple. They can be metal spring clamps, screw clamps or the like. The screw clamps can be set for enough pressure to literally crush the nipple if that's what the top wants, but in general use, they start off tight and as the tissue compresses, they get loose. The spring type usually don't exert as much pressure as you can get with a screw, but they stay tight because as the tissue compresses, the spring keeps the clamp tight.
A tit press is usually two pieces of wood or heavy plexiglas held together by two (one on each side) or usually three screws (one on each side and one between the breasts) with wing nuts that clamp a good portion of the breast.
I prefer the presses that have three screws because you can set the center one first which helps positioning the press, followed by tightening the side screws for the real pressure. Ultimately the three screws are tightened for the most pressure. I like to use these on others more than I like them on myself.
NOTE: also see my posts at the beginning of this thread.