There are no "best" positions. There is only what you like best yourself, and what the mood is. For us, there is the "simple jerk"... which is her playing with my cock, some BJ action maybe, then I jerk off while she rubs my legs/chest etc. Then there is what I would call "let's watch" method... lying across from each other usually with legs intertwined, and propped up in a sort of sitting position... and mutually masturbate while watching each other. Then there is the "Cum on her" method. I will straddle her abdomen, so my cock is basically above her tits. She rubs one out while I jerk off above her. I cum on her breast usually. This method is probably my favorite. It has a dominate element to it, and she also likes to be in a submissive position.
When we dating and not actually having intercourse, mutual masturbation was our release. It was always in a car, in an awkward position. Now we enjoy it as part of foreplay. Sometimes we are lying on our backs, side by side, watching a movie and fondling each other. Other times, I'm standing by the bed, using my fingers or a vibrator/dildo on her while she gives me a hand job. In both cases, I make her stop before I cum, but I sometimes bring her to an orgasm and finish her off inside. It's fun any way you do it.
one of my exgirlfriends like when i was 18.... we used to masterbate together with her friends there would be like 3-5 of us getting off together... since then it's only happened with a few girls but nothing on a regular basis... i think it's pretty fun to do
Mutual masturbation can be a great time. Looking into your SO's eyes while building towards orgasm. Once you are both turned on it is easier to communicate about kinky things you haven' talked about. As for positions I tend to prefer the lotus, but then it is way to easy to stop masturbating and fuck. No insider secrets although if you are talking investments I recommend "Snowflake"
One of my favorite ways to pass time on a dreary winter weekend. We get out our box of toys and masturbate together watching porn. So much fun!