I downloaded a book called "How to make love all night"
It's got load of stuff about the PC muscle and other workouts you can do to help sexual performance.
They say to 3 lots of 20 reps a day on your PC muscle, holding each clamp for 2 seconds. Do this for about 3 weeks to build it up and then start doing "super Clamps" where you tense the muscle really slowly and hold it for 15 secs.
Ive been doing it for about 2 weeks and so far so good. I think i had a semi-ejaculation the other day as well so hopefully with a little more practice nothing will get out that i don't want to!!