Joining a New Ball of Yarn

From BluWiki

When knitting flat it is best to make your joins at the edge of the garment. That makes the woven in ends less noticeable. You could even hide them in the seem.

The easiest way to do this is to hold the two strands (the old and the new) between the second and third fingers of your left hand to hold some slight tension on them and then knit as if it were one continuous strand.

The video demonstrating this is here.