I had a 15 year old concrete floor, no major cracks but a couple of nasty oil stains (previous owner naturally

). I used a grease cutting dish detergent with hot water to soak up the main portion of the oil stains. It took about 7-8 repetitions with a whack of paper towels. The whole concrete floor was swept, given a blast of diluted muriatic acid, rinsed with clean water a couple of times and then, when thoroughly dried, the adhesive was trowelled on.
EDIT I just remembered that I rolled on a thin coating of Armstrong Primer before the adhesive was put down.
That was the extent of my prep. The muriatic acid was probably overkill but it isn't expensive and I wanted to do this job just once. Its been a year and a half, into my second winter where our daily temps hover around freezing or just below, and my garage is insulated but not heated. The coldest my wall thermometer has indicated in there was 40F. There have been no issues with lifting, cracking or peeling of any tiles. The summers here regularly register temperatures in the high 90's.
These photos may help a little.
http://www.cardomain.com/ride/281382/14