Materials - Synthetic or Real?
There are many synthetic (faux) wood beams available. Some products are made of dense Styrofoam or PVC plastic. Others are made from recycled paper or wood products. Nothing beats the look of real wood.
Check in your area for purveyors of architectural ornament, or demolition contractors. You might be able to buy remnant beams from other structures and have them modified to suit your design plan. You can also construct new beams from scratch, either by box framing them or having the local lumber yard mill them from large stock.
Weight - Heavy or Light?
Always consider the actual weight of the beams. Determine the method of attachment required to hold them in place safely. Light synthetic beams require simple attachment, often into drywall alone. However, you will need to get solid attachment into the ceiling joists for heaver wood or box constructed beam.
Lighting Impact Dark Colors or Light Colors?
When you design your room plan to incorporate beams, consider the impact the beams will have on the room lighting, as beams tend to cut off ambient light and create shadows. Also, keep in mind the color of the finished beam. Dark colors tend to create the feeling of a lower ceiling, where as light color tends create the feeling of more height to the room.