Yep, sex is more of a "nudge" thing for labour, than a "inducing" thing. You have to be ready, and then sex can help just tip you over the edge.
Orgasm causes oxytocin to be released in greater amounts. Oxytocin is the same as syntocin, which is one of the things used to induce/augment labour. Nipple stimulation releases the same hormone. It also triggers milk let-down, and uterine contractions, which is why you can get a let-down during sex after baby, and why you get after pains after delivery when you nurse. Oh, and braxton-hicks that are pretty hard, maybe even a little painful are normal after sex. No worries.
Semen contains prostaglandins which help to ripen the cervix and begin the softening, dilating, and effacing. Apparently to have a "real" effect would take like a bucket load of semen, but every little bit can help when you're trying to start labour. Again, it won't trigger anything until your body is ready.
As far as positions go - I don't think there are any no-no's. Just whatever is comfortable. Some positions may be more likely to allow air into the vagina, which can cause a slight risk of an embolism (or something like that), so you might want to avoid any position which does that (who needs that anyways!). Other than that - you should be all fine to do as you please, unless your doctor has given you specific restrictions, or your waters break.