Tip 1: Less is Best
No flashy jewelry, heavy makeup or anything that drown out your true essence. Lifestyle pictures are perfect for those items but a head-shot is a timeless view of YOU, so show that.
Tip 2: Iron it out
Wrinkle free clothing that fit well is your best way to go. Bring several outfits so your photographer can gage what looks best against lighting, backdrop etc.
Tip 3: No words.
Avoid name brands with logos, large prints and especially writings. The headshot is about YOU not the manufacturer.
Tip 4: Assume Model Behavior
Always consult your agent before making a final selection. Most images liked by models are generally not preferred by the agency. Most models who pick their own images are refered back by the agency.