I've stressed before how important your posture is not just when exercising but all the time; bad posture creatures tightness in certain muscles and makes you unable to engage others. Your pelvic posture is incredibly important to keep you upright, avoid back pain, allow you to have a strong functional core among many other things. For women in particular it becomes important for pelvic floor engagement etc. These days having a big popping bootaaay is all the rage, but sticking your butt out can create real problems for your low back and your posture. As soon as you place yourself in this anterior pelvic tilt position it becomes really hard to engage your core. This makes activities such as weighted squats really problematic, as you risk your low back health. 

Exercises to help engage core and correct poor pelvic posture
 1. Pelvic Tilts: Lay flat on the ground with a round in the low spine, slowly rotate your pelvis so there is no longer any space between the floor and your back. Brace core, breath deeply for 4 counts, bring back up. 12-15 repetitions.

2. Cat Cows, you should have a certain amount of mobility through the spine, work on rounding the back in the mid/upper region, then creating an exaggerated rounding in the lower region. Go back and forth between the two movements 12-15 reps. 

3. Deadbugs. Bring arms up in line with shoulder, bring knees in towards the chest, really draw in through the core, bracing from all sides and focus on breathing out as you lower one leg at a time, alternating. 12-15reps. 

 4. Bird Dogs: Knees under hips, hands under shoulders, bring leg arm out and right leg out, ensure back is nice and flat and you are in control of this movement. Hold the extension, bring in together. To ensure you're not wavering in this posture you can place a yoga block on the low back. 12-15reps per side.