Is there a way to prevent queefing (vaginal farting) during certain sexual positions (doggy style)? thanks..

Not usually: Passing some air during sex, particularly in positions where the pelvis is raised (like "doggy style") is normal. Air normally flows into the vagina, and intercourse and sudden changes in position can force some of the air out. If you have had surgery in that area, intestinal problems like crohn's disease, or notice this often, without sex, see your gyn. Try another position!