Take the time to learn to carry 3 glasses. Better yet, 5. Trays are fine if you're comfortable with them (and possibly required at your restaurant), but using your hands to carry 3-4-5 glasses out is simply faster.

This is a surprisingly useful skill in daily life too....

I don't believe there is ONE way to do this so you'll need to just play with it some until it clicks. This video would be a good place to start though.

I've had many requests for a video on how to how use a tray -- unfortunately, I can't offer one as I myself never use a tray. I prefer to just make multiple trips as I can move pretty fast, and honestly, trays make me a bit nervous. I never felt secure with one, and I've seen customers try to grab drinks off a server's tray in an effort to "help" which is nice of them but neglectful of certain principles of balance.

I can offer this as I once knew a guy that was gangster with a tray: I asked him how he did it, and he answered with something to the effect of, "just be confident and own the tray."

Regardless, any server should know how to carry 3 glasses in one hand.