TIP 1: When shooting outdoors (or indoors even) remember to also try composing with the camera tilted into a portrait mode, i.e. on its side. This can often ‘tighten up’ a composition and make it more interesting.
This contortion becomes easier if your SLR camera has a battery grip as these usually come with a shutter button which, when the camera is turned, ends up on top (depending on which way you turn the camera!).
If you have plans to sell your work through an agency, around 60% of sales are from portrait format images – picture editors love them for magazine covers and find them easier to crop anyway. Remember to leave some space top and bottom for titles and captions if this is a route you want to travel down.