Don't force the child into making it, rather let them watch you sign something, then WAIT for their attempt to imitate you. You may be waiting up to 10 seconds. If no sign imitation is used, model it again. This time you may use a partner to stand behind your child and physically help your child produce the sign, or gently help the child make the sign, then give them what they requested. Don't expect too much too fast from every child.
All children are different and learn at different rates.
It may take many models before the child begins to imitate. Thus, the waiting period before helping the child is VERY important. Also, you may model many signs throughout the day, but try to expect only what your child is able to produce in return. If you wish to view more signs, I would suggest borrowing a CD-ROM or video from the library, or taking an adult education class in your area. If you have questions or concerns regarding your child's rate of speech development, please seek assistance form a licensed professional.