While I agree that the easiest way to get laid is to boog a wg, there is a certain thrill of the chase in "extra" extras from parlour girls. I'd say the best approach is to build a rapport with the girl from the moment u meet her. Keep it fun and later things can get a bit more physical. Also u are going to have to go with b2b, I find it impossible to get "extra" extras if she wont even do b2b.
Once you achieve a rapport, use her greed:
Lets say you've paid her for the b2b and she's busy sliding herself all over you - casually hold out a twenty and ask her what she would like to do for it. I'd say this works 50% of the time for BJs. FS, maybe 20%, but you will need condoms and you'll need to be ready to to wave a second £20. I've once had FS without the extra payment. But this was like 1 in a lifetime. Also it was BB (We both got carried away). I dont recommend it and shat myself untill I got the GUM green light.