Always use all applicable TCA specifications.
Concrete
First, you need to determine if the floor is level and if there are any surface cracks. If the floor is not level, leveling products are available at your local tile center. Minor cracks will need to be filled with concrete repair products, or some type of crack isolation membranes. Always refer to the TCA handbook and ANSI specification manual for proper techniques to prepare floors. Next, check to see if the concrete is clean. Concrete should be free of grease, oil, waxes, curing compounds, sealers and other adhesives. These contaminants will affect the bond between the tile and the slab. It is recommended that the concrete should have a textured surface. Please refer to the TCA handbook for surface texture prior to installation.
Level, condition, cracks, clean, You need to address all these factors. Any expansion, etc. joints need to also be expressed in the tile.
Any real tile guy should be able to quote tca specs. If they say "what's tca?" then run away and find someone who doesn't scratch their head when you mention it.
Here are some good pointers:
http://www.thetiledoctor.com/install...urfaceprep.cfm
Click the red section links in the left column