Ditra membrane from Schluter allows you to go over OSB, and only ads about 5/16 of height. It is also an uncoupling membrane, which allows for some shear movement. It costs aprox. $1.50/ft, but is well worth it, as it can go in faster than plywood, and provides an excellent substrate.
I installed 24"x24" Quartzite tile purchased from the set surplus of "Night at the Museum 2" in North Van a couple of years ago over Ditra XL. Before the Ditra, I installed 3 zones of electric in-floor heating, and everything is performing as expected.
I don't install with out anymore, as it solves too many headaches, and it's the only product with a 5 year warranty, AKAIK.
As others mentioned, stay clean as you go, because life sucks when you're scraping dried grout all day. I usually keep a dozen clean, rung-out sponges/clean water pails on hand when setting and grouting, although if you over clean grout, you can end up with efflorescence.
Oh, and when laying out, you should never end up with less than a full tile.
To eliminate the small piece, add a full tile to the scrap and divide by 2. That is your starting cut (and should be your final cut). Don't forget to include your grout width when calculating for cuts. I can always tell a rookie's layout when you see one side of a room starting out with a full tile, then a 3" sliver row running along the opposite wall.