We stock 3'x5'x1/8" panels of JetBacker, a magnesium-core board that can be cut with a box knife. 1/8" Jetbacker is an easy way to add height behind a thin mosaic or tile so it matches up to surrounding material.
In the above cross section, you can see how the larger trowel required for the thicker, larger tile adds 1/2" of height. To match this, we added our 1/8" board with a layer of thinset above and below. A thinner bed of mortar under the small mosaic tiles prevents it from oozing up and getting into the grout lines.
Using this technique, you can achieve smooth, flawless floors, backsplashes, and walls, as seen below.