How do you prevent sand from leaking into pool?

It sounds like the lateral pipes in your sand filter are broken or you have too much sand in the filter. When we first got our pool I vacuumed with the filter on backwash. I found sand settled in the steps when I turned it back on to filter. I was later told by my pool store that you vacuum on waste and then use rinse before turning back on to filter. That stopped my problem except for dirty feet getting into the pool! both previous answers are correct to a point. the laterals or stand pipe could be cracked or broken' vacume on filter and then when finised backwash well and then rinse for aprox. 30 to 40 seconds and then go back to filer. make sure you rinse before going back to filter. Don't forget to turn off the pump before rotating the back wash valve! K