To tell the truth, the easiest way to get a smooth surface is to work at it as you go, while it's wet.

Once it's dry, sanding is the only way I know.  If you have one of those electric units (like a Dremel Moto-tool), you can use that VERY CAREFULLY.  If you have the energy, wrap a piece of sandpaper around a smallish block of wood, starting with a medium grit and working to a finer one.

If you're using papier mache pulp, let it sit in the open until it's semi-dry, then smooth it with the back of the bowl of a spoon.

When you're applying paper (moist) strips, keep your pieces small (relative to the size of the project). Every time you have a fold in the paper, it's going to dry as a ridge. When you add paper over it, it will form a lump.

It is difficult, curbing my haste and controlling my impatience, but I hate to sand!

Sue