I made lasagna in a deep dark roasting pan and froze it.  Now I am going to have to reheat it. How long in the oven? 
Two suggestions, one practical, the other probably less so.  1) Hold the cup directly, instead of the plate the cup is on. I don't know if that's allowed by health code or restaurant policy, but if it is, you get much more stability from cupping the cup in your hand than balancing it on a small plate. If you're already doing that: practice will get you there.  2) A good cappuccino should be fairly stable. I know there's disagreement about the proper way of layering cappuccinos, but the best ones integrate very finely textured foam into the milk, instead of just layering coffee - milk - foam on top of each other. Milk foamed this way is more stable than regular milk; I've carried cups filled to the brim up the stairs without any spilling. But: this is a barista-side issue, and to be honest, I wouldn't touch it ... the barista is going to make the cappuccino how s/he feels it's best, and being told to do something differently won't go over well. 