2. Make sure your keys are seating properly.
3. Think "hard boiled egg" with your mouth - you need an open, resonant oral chamber to ensure the reed is vibrating fully. You also need a well-soaked reed that isn't too hard or soft for the player. I usually use an "OOH" shape for my mouth when playing the lowest notes.
4. Make sure the fingers aren't hitting any extra keys. It's very, very easy to vent an extra key here or there and then you get squeals or wrong notes.