"[T]he plan sponsor should get rid of all revenue-sharing payments. These create insidious conflicts of interest.... A fixed fee should be negotiated for services in establishing an Investment Policy Statement ... and for assistance in selecting investments.... [T]he plan sponsor should ascertain the process for selecting asset classes and then for selecting investment products in each asset class, given the needs of the plan, its participants, or the fund.... Follow that process -- with diligence and thoroughness."