Not brushing long enough: The average person only brushes their teeth for 45 seconds. 2 minutes can feel like a really long time!
Choosing a toothbrush with hard bristles: Hard bristles can cause abrasion and damage to the outer layer of your teeth. Choose a brush with soft bristles and brush gently--you're removing soft plaque! Use pressure light enough that the bristles do not splay out
Not brushing the gums: your gums are scalloped shape, and your brushing motion should match that with small circles. You want to brush debris out of the small pockets between your teeth and gums.