For side to side (along X), make sure the laser is tram (square) with the platform - I use some 1-2-3 blocks as spacers: Carriage Tolerances - Unusable Over Distance >75mm From Center
I also shimmed under the platform with tape or any other shim material. Others have used copper washers.
A more long term solution to address the bed flexibility is this: Y Axis Rattle - Proposed Solution