Selling appetizers can be very challenging. Often times, patrons will hold off ordering an appetizer because they know they won't finish all of their meal due to large portions. Although I'm not as bad as I used to be, I'm still a pretty picky eater, so whenever suggesting an appetizer or side order to a guest, I always throw that tid bit of information in there. "I'm normally a picky eater, but our garlic breadsticks with cheese are to DIE for." Patrons take this as, "Well if she likes it, it must be good!" I can't say it works everytime, what pitch does? But most of the time, it does!
As a manager I often conducted sales challenges with my staff. A friendly contest of, "Whoever sells the most apps in a two week period wins ..." Friendly competition usually builds camaraderie and a sense of team.
As far as a pitch. I think that every table has to be read and approached individually. I ask my servers to try to sell a shared app if they've encountered the small appetite guest.
Appetizer sales depend greatly on the rest of the menu. The larger the portions of main dishes, the less likely you'll sell appetizers. Including salads with entrees also reduces appetizer sales.
That said, as far as pitches go, the most effective way to lead a customer toward any dish is to simply make a strong recommendation in a way that doesn't sound like a sales pitch. For example, a server simply saying, "The roasted polenta cakes appetizer we have tonight is AWESOME! You HAVE to try it!"
Guests like to be lead by someone "in the know". Make sure your servers are "in the know". That means training and constant sampling.