Maybe you can't whistle at all. You might be a hell of a snapper, but that's not really going to help you out in this situation. Rather than try to practice whistling in a sexy, seductive manner like putting your lips together and blowing, you can just flail your arms around like a big idiot.
Unhinging your shoulders and waving your limbs in all directions as fast as possible like you're one of those inflatable air-dancing tube dolls is a great way to grab a cabbie's attention. Always remember to stretch first and forget that you are an adult human being in a modern urban setting who once had dignity. Incoherent yelling increases efficiency by fifteen percent.
Try to imagine you're not flagging down an automobile but rather landing an airplane in thick fog. Just be careful not to use actual FAA hand signals or a 747 will land on your face. And they won't go to the Bronx, either.