Linking/attaching eyes to animated character

Hi,
I'm new to animating and am having problems with my character's eyes staying with the model when I animate the character. I have made the body, added a biped and a skin modifier to the body mesh. The eyes are separate objects so I used Select and Link to link them to the body mesh, but when I animate the biped the body moves but leaves the eyeballs behind. I also tried linking the eyes to the biped head bone, which did make them move with rest of the model, but when I display the biped as boxes the eyes display as boxes too, so doesn't seem the right way to do it. Can anyone advise me on what is the right way to attach/link the eyes to the model? Many thanks!