The "dry shake" is a technique that bartenders use to emulsify the egg in egg white drinks to give them a nice foamy head.
Before adding the ice, they put the egg white and other liquid ingredients in a cocktail shaker and give it a good shake. Then they add ice and shake again.
Many bartenders also put a spring into the shaker with the liquids to help the dry shake along. This spring is taken off a Hawthorn strainer and used. This is a pain in the butt though.
Audrey Saunders (owner of New York's Pegu Club), speaking at a seminar at Tales of the Cocktail, mentioned in passing a different tool to use instead of the Hawthorn strainer spring: a cat toy.
I haven't tried it myself, but it seems there are a lot of good options out there. Some are rubber balls (would a gumball machine SuperBall work?), some are whisk-like...
And some are even shaped like springs.
Have you used these? Anything else?