From http://gourmetfood.about.com/od/drinkrecipes/r/wtrmelonmartini.htm

1 cup watermelon juice (press watermelon through a sieve or cheesecloth)
1/2 cup (4 oz) Vodka
1/4 cup simple syrup (sugar)
juice of 1 lime
3 tablespoons salt (optional)
3 tablespoons sugar (optional)
ice
Watermelon slices, for garnish (optional)
Preparation:

Makes 2 servings.
Mix together the sugar and salt if using. Wet the rim of a chilled martini glass with a piece of watermelon. Dip the rim into the sugar and salt mixture. Repeat for other glass.

Place the watermelon juice, vodka, lime juice, and simple syrup into a cocktail shaker. Top with ice. Shake well.

Pour contents through strainer into martini glasses.

Garnish with a wedge of watermelon if desired.