Have an arousal scale of 1 to 10, where 10 is orgasm. Tell each other, where you're at, during the act, then adjust to bring yourselves to the same level.
The main thing you should do to come together is to relax. The more at ease you both feel, the more likely it is that you'll enjoy the ride - literally.
Don't just fall into the missionary. The basis of “synchro-gasming” is control over your climax. Try to vary positions to synchronize your arousal.
Therapists suggest watching your breath when striving to “synchro-peak”. Take deep breaths with each slow move. If your breathing falls into harmony, your bodies will too.