If the model can be positioned close to a bit of scenery, you can usually hide a metal rod under the model to hold it to the scenery. You can obviously see the rod if you pick the model up, but when its on the table it can be hidden. If its more of a flying monster then you can usually model such things landed or perched on scenery, or in the process of lifting off or landing (go go dynamic posing). What kind of model are you thinking about?
If you have access to cryogenics and high temperature superconductors then you can magnetically levitate things, but thats not usually too practical for gaming with
I suppose a viable method would be to create a frame with the base scenery and suspend the model from above using thin thread, but unlike for model aircraft on the ceiling the thread would be pretty obvious, even if it was translucent fishing line or similar.