Steve, make sure the floor is clean as possible and
then clean it again. Use a paint roller to lay some
liquid latex. It's cheap insurance. Then, what I do
that has paid off very well is to flooring glue. Even
though it is self stick, the flooring glue makes for an
easier job and better results. Use a 1/16" notched trowel
to lay the glue and then install the tile. This will help keep
the tile from shrinking in the future and leaving those gaps.
Also, it helps when laying the tile. If you accidentally put the
tile in the wrong place or off a 1/4" it will just slide into place.
Make sure you have a wet rag with you.
Lou