First make sure you're flexing the right muscle, the bet way to find the muscle is to completely stop while you're pissing. Flex the muscle as hard as you can and completely stop the flow. If you're flexing the wrong muscle you won't get the type of results you are looking for.
I recommend starting off doing about 100 reps, flex the muscle hold for one second then release. Doing it with rythym will help too.
Next do 25-50 flexes, but hold them for longer maybe 10 seconds at first, release for a few seconds and then flex it again.
Finally, practice doing fast pulses. Flex the muscle completely and release immediately. I like to do 50-100 at a time of fast flexes.
I recommend you start doing a routine like this once a day at first. Increase reps and develop your own flex patterns. Don't overdo it though, you can overwork it just like any other muscle. The effects will be more noticeable if you do it within a few hours of sex/masturbation/whatever you might do. The most important thing is to be consistent, do it everyday unless you start to get sore. | 
