It sounds stupid to say, but you need to shake drinks with really cold, solid ice. Certainly, it should be in pretty big chunks.
You're basically using the ice the way a spray paint can uses that agitator ball. The ice will slam back and forth in the shaker as one mass. Once it starts tinkling rather than slamming, it's time to pour. If you're giving it some effort, 10 seconds should be good. Keeping really cold ice will limit the amount of melt, but the extra water becomes a part of the drink's flavor too.
My personal shake pattern sounds like "Wipeout" by the Surfaris. Fierce sixteenth notes accented on the 2 and 4. I suppose I could try the hemiola phrase from "West Side Story's" "America" (duh-da-duh-duh-da-duh-DA-DA-DA) some time.
Just make sure to hold the cap on the shaker to avoid decorating your guests.