I was worried about DS2 not speaking much at 15 months & made a conscious effort to teach him words. I haven't done much sign language with either of my boys (although I have friends who rave about it). Instead, I just focus on particular words in our day-to-day interaction. As some of the others have said, I would repeatedly use words (eg apple, drink, up, down, please) - but I wouldn't make them into sentences - just focused on the word/s that I wanted him to learn. I also found that reading books a lot & pointing to/talking about the pictures helped him to learn words (particularly animal sounds and names of other things - car, ball, truck, water, tree, etc).

You may find this blog post helpful (written by a Speech Pathologist)
http://www.theusefulbox.com/2012/05/...esnt-talk.html