Crappers, you wouldn't believe how many floor voids are quietly rotting away out there with this problem.
The "secret" is to get it as watertight as possible BEFORE the tiles go on. It's Ok to let the tiles/grout do all the work, but they're vulnerable to the odd crack/gap in grouting etc. Then the whole thing fails.
I like the "L" shaped type of tile trim. One leg goes up the wall. The other then drapes down over the tray. A good bead of silicone in the joint between the wall and the tray first ...... then the tile trim.
Tile down over that, and finish with a neat silicone bead along the bottom of the tiles.
Also, the most vulnerable area... a silicone bead vertically in the corners,