Typically using the !admin stalkword in #wikipedia or #wikipedia-en is your best bet, I think.
I think you're also more likely to get a response if you indicate what you're looking for help with. "I need an admin," may be less effective than "I need an admin to block a vandal," or "I need an admin to help with an OTRS ticket. Can PM." Some people want to know what they're getting into, or maybe it just gets their attention better; I couldn't say for sure, it's just a trend I've noticed, off and on.
There was a #wikimedia-admin channel at some point, designed to be a one-stop-shop for admins from all Wikimedia projects; not sure if it's still active.
-Luna