What are the best sex positions when not trying to conceive?

Doesn't matter: There is no preferred sex position that will protect you from getting pregnant. If sperm penetrates in the vagina, you are at high risk for conception. Any position that involves the presence of penis in the vagina is "high risk".

Any position fertile: Any sex position is fertile and no position is "safe". Semen is sticky, and sperm can swim, so any intercourse position where the semen is placed anywhere inside the vagina can get a woman pregnant. The penis often is still moving during and after ejaculation, and so will spread the sperm all over the vagina and cervix.