What is the best way to reinforce the bathroom floor of a mobile home so that the washer doesnt vibrate so much?

I have dealt with that exact problem a couple of times. First time I added a sheet of 3/4 plywood to fit under washer and dryer so both stood higher off the floor. I then made 1/8 deep depressions where all the feet fitted. At least then they didn't vibrate out of place. In the end I decided that the only really effective solution was to adjust the load of the washer before it went into 'spin' mode.