Tamping the bottom with a 12"x12" hand tamper. This is one of the things I wish I could do over. I went over the whole pool floor three times with the tamper. I sprayed water lightly on the sand and got it completely solid to walk on.
However, the pool floor did settle somewhat and you can feel some depressions in the liner. In hindsight, I would have either trowelled vermucilite in or rented a 300 pound plate compactor and went over it a few times.
Of course, my Wife tells me I'm the only one that notices.