Once she starts working hard, try to keep questions to a minimum and use questions that only require a yes or no response. The less she has to think and make decisions, the easier it will be for her to labor.
During contractions, be careful to avoid doing things that will disrupt her rhythm such as chatting with others, asking her questions, abruptly starting or stopping any kind of massage or touch.
Low and deep vocalizing can be very helpful. These sounds can seem animalistic and/or sexual in nature. The less inhibited she is, the smoother it will go. The same goes for clothing. As labor progresses, she may lose her modesty and start shedding clothing. Trust the process.
Stay present and be with her--both mentally and physically. Resist the lure of the hospital monitors, TV, phone, and especially the clock!
Remember: you know her better than anyone! You are going to do great!