My contractor who first installed my slate floor in the kitchen recommended
Miracle Sealants 511 Porous Plus Sealer Professional’S Choice.-V168*
After 16 years the floor looks brand new! It's not cheap, but the sealer is guaranteed to last 20 years. And it applies easily with a white washcloth, which I find lets you do it very evenly. Though the instructions say they sell an applicator. If you want to only seal your floor every 20 years, invest extra money for a professional grade sealer.