1) Don't get something for nothing! It may sound like common-sense, but if your child wants an item or activity, make sure he has to do something communicative to get it. This can be as simple as a point or eye contact, but can build up to ...vocalizations or signs.
2) Completing the line: Does your child have a favorite song? Or does she like games like Peek a boo or chase? Start a phrase or song line and have her put on the last word (or even a sound) to continue the game or song. For instance, you could say: 'Ready, set...' and when she says 'go', she gets chased! Pretty fun...
3) Repeat. Repeat. Repeat.: The more chances to request, the more practice your kiddo gets. He wants juice? Fine. Just give him a little bit. Enough for a few drinks. Then hold the juice container up & have him request again. Same with an activity like a video game or coloring. Have the child stop after 5-10 minutes & request again.
Pick one of these strategies & give it a try tonight!