Hairstyle for a triangular face shape?

Try

Shorter hair that balances prominent jaw line. Styles that are full at the temples, taper at the jaw.  Off center parts. Wedges and shags look great on you. You need lots of layers to achieve fullness through the upper part of your face. By wearing styles which are full at the temples and taper at the jaw line, you achieve a balance which can be remarkable, in accenting features otherwise understated. Try tucking hair behind your ears as this will draw attention to your eyes and add width in this area.

Avoid

Long, full hair styles that draw attention to jaw line. Center parts—off-center is more flattering. 

Too much height at the crown.  You want to stay away from putting most of the weight of a haircut at the jaw line and below. This will give the appearance of added weight to the face. 

If going with long hair, it should be kept tight at the nape.