Medium to longer styles with soft edged bangs and a graduated shag or long layers will give the face a longer and more slenderized shape. Stay away from blunt bangs and remember that shorter cuts can definitely work on round faces but make sure that the perimeter doesn't hit at the widest point of the face.
Longer layered bangs are a great way to draw the eyes to the cheek bones and break up a square face. Medium length styles with some wave and movement will be best to soften square jaws. Avoid center parts and very long straight styles.
Heart Shaped Face:
To balance out a heart shaped face, it's best to cut soft bangs that hit around the arch of the brow. Just make sure to not cut them too short! This will balance out the wideness at the top of the face and draw the eye down. Short pixie cuts are also ideal for heart shaped faces.
Your oval faced clients can wear a variety of styles. For medium to long hair, make sure the layers are hitting at the point on your clients face that you would like to emphasize. Also make sure to not let your clients hair get too long as this look can drag down an oval shaped face. Short cuts can also look great, try blunt edgy bobs and cropped styles with longer bangs.
With a triangle shaped face you are trying to narrow the jawline and bring the eye upward. Graduated bobs with tapered layers look best for this face shape, slimming the jaw and adding fullness to the top and crown.
Consider all Factors:
Keep in mind that these are only examples of looks that will maximize your clients bone structure. The rules don't always apply to everyone so make sure to also take into consideration hair type, texture and lifestyle before deciding on the best cut for your clients.