You're in a club and you see a girl leave her drink at the bar when she goes to powder her nose. When she comes back, instead of actually spiking her drink, rush over to her and scream "DON'T DRINK THAT!". Explain you saw some middle-aged guy slip something into her drink and you just wanted to make sure she didn't end up Fritzl'd.
Next, slyly suggest she needs a new drink and that you would be willing to buy it her. How it goes from there is up to you.
You hit a dry patch, dude?
Fo' sho, looking to pull off a Roofie Rescue tonight.