If you opt for a clamp-style bike stand the the clamp itself is really the heart of the system. The best clamps will grip well without needed excessive force.
it's worth reiterating again that you should always clamp the seat post wherever possible, but for a quick fix you can get away with simply resting the toptube into the lower jaw and let gravity hold it in place.
ideally the jaws will be lined with soft rubber. Not only does this avoid scratching your pride and joy, but it also increases the purchase for a given clamping force, making things more suitable for delicate tubing should you have to clamp the frame.
most of the bike stands listed above will have daws with some degree of fine adjustment, which also helps to keep things safe.
If your stand has hard plastic jaws then line with a soft cloth, or wrap the jaws in old inner tubes to achieve the same effect.