i have a shit ton of pornos in my dvd collection, many of them about squirter chicks
i have one in particular by Seymour butts and he actually walks you through doing it
i tried when i was in collage to about 6 girls and like 2 of them could do it and they didnt know they could
let me tell you its helluva forarm workout!
they get to the point where they feel like they have to 'pee' and i just tell them to let it all go. thats the part where its hard for them, to get them so relaxed, many of them feel embarrassed.
also you have to put tons of pressure with your fingers UPWARDS (or down if they are in doggy position) and work that spongy feeling area (gspot)