Yo mate!
i am in a relationship as well. I have never stopped doing pickup, although i must say that there arent any closes of any kind anymore.
I got 2 ways of doing it, but both not that direct anymore.
The first one is where we are both total strangers. I usually do the approach, and i let a connection grow. I basically do the same as when i would be a single (yeah, sometimes heavy flirting). Then i usually go out for a drink. For me its a chance to see who she is and i try to connect on a deeper level. Usually that is already enough. I dont tell i am in a relationship in the beginning, and after some fun, i usually break it out. My girlfriend knows i am doing this, its ok for me to go out with total strange women.
It happens sometimes that people already know me, or know things about me before i get the chance to meet them. This is either because of facebook or because of the work environment, or friendships etc. I usually start convo when we first meet, then i find out she already knows me. if the encounter is actually fun, i might ask again to go for a drink (yeah, i have quite a few drinking buddies now

) or anything else that is fun to do. it aint always a drink. Sometimes they will come, others will refuse because of the relationship. Then i need to do some more work, which sometimes involves a conversation or two on facebook. That is usually nothing special, its just to convice that it is really ok for me to do this.
dont know if it was of any help for you
cheers!
EDIT: oh, and when i say that i do the same like if i would be single, i really mean that. I try to keep my skills sharp and try something new. You will see that it is much easier to do if you really have nothing to lose.
And when you are actually successful, its easier to push them into the friends zone.