Buy the smallest quantity of DE powder you can; add a quart full to the skimmer with the pump on. If you see the DE shoot back into the pool via the returns . . . your filter needs work; dirt is bypassing the sand (or maybe, there's not enough sand). If the DE does NOT shoot back into the pool, your filter is probably functioning correctly, however the DE will cause it to build up pressure more quickly, until you backwash.

Generally speaking, if you have a correctly functioning sand filter, you should backwash only when you need to (4 - 6 psi increase) or 1x per season, which ever comes first.