Unlike scaling a multipitch climb, a multi-tiered cake can be secure, well supported and predictable. One of the main ideas around stacking tiers is to make sure the upper layers don't sink into the lower layers.
First you want to assemble and ice the individual tiers on cake rounds, preferably ones made of plastic that look like corrugated cardboard (they're lightweight and durable) that are about the same diameter as the actual cake.
Before you stack the tiers, mark an outline on the bottom tier where the upper tier will go. Use the pan in which you bake the layers and gently trace the outline onto the icing or use toothpicks to mark the outline. Within that outline, insert plastic drinking straws vertically into the cake (yes, this really works!). Cut the excess straws off flush with the surface of the cake. Be sure to insert a straw into the centre of the cake. Rough guidelines: for a 9" cake, use 6 straws plus 1 for the centre, for anything bigger, use 8 straws plus 1 for the centre.
Gently place the next tier on, using a large pancake turner or other inflexible kitchen tool. Continue with any additional layers. Touch up with extra icing (always good to have on hand!!) or decorate with fancy icing or piping.
In terms of transporting an assembled cake, I recommend icing the layers, then transporting the cake unassembled and tiering it where it's going to be served. I'm not gutsy enough to transport a 30 lb, 18" confectionary masterpiece in the car!!