Using flashcards is a pretty good method, but with 2nd graders any hands-on approach would work. In kindergarten, our teacher would ask us to bring in an object that starts with a "b" or a "g" etc. then we'd bring the object and show the class and tell the class what it is.

I teach older students but vocabulary is still a problem. For flashcards you can place the flash card on the object...make it into a matching game serveral different ways. Games and songs. Using lots of pictures with the words helps. Having them create their own picture of the word. There is just a ton of ways to teach vocab. But there is not a best way...kids learn so differently.