Do This:
Fig.3
Make sure that your knees don't go past your toes (see Fig. 3). Be sure to maintain proper posture without any leaning of your back. Proper breathing is also emphasized during this lift. *Note: keep those abs tight!
Not This:
Fig. 4
Fig. 5
Notice the knees pass the toes (see Fig. 4) and the back is hunched over (see Fig. 5).