Great question.
I know for me I always try to stay tilted up a little for about 10 mins after we've had sex (put a pillow under your bum or just bring your knees up to your chest).
Having an orgasm helps to because it opens up the cervix.
Certain sexual positions help, where the man had 'deep' penetration.
Sorry...not exactly the answers you were looking for but I just wanted to say that besides the CM there's other stuff you can do to make sure the *swimmers* have a good head start