If you are completely tired, sometimes you may fall asleep during meditation and that is ok. If not for the mentioned reason and you are falling into sleep frequently, You should change your food habits and start doing yoga or some workouts.
Sattvic foods (You can find lot of literature with regard to sattvic foods. Please google it for reference) will help you maintain your posture and bring in the required alertness. Change your diet and avoid all junk foods. Participating in Satsangs will also help.
Increase in the Sattva will bring in the required alertness and will help you not to fall asleep.back