What are the precautions needed when tiling over existing tiles in a bathroom

First check to see if the tile is cracked or lose
If it is I would tear it out and start from the sub-floor up
If it is solid I would clean and scuff up the face of the tiles then apply a mortar latex additive to help provide a bond let dry and tile as usual
Get r done

Porcelein or vinyl, what kind of tiling?

If you do tile over tile these will come in handy when you try to reseat the toilet with all the new height.

Jones Stephens Toilet Bowl Flange Extension, Stackable Extension, Model C88-001 (usahardware.com) (http://www.usahardware.com/inet/shop/item/81540/icn/20-612432/jones_stephens/c88_001.htm)