In rough order of importance:
Open eyes (with glasses)
Alert mind
Working ears
Bike in (reasonably) good mechanical condition, including:
- Good fit to rider
- Good brakes
- Good lights in low visibility conditions, including darkness
- Tires (usually) aren't bald and are inflated correctly
- Chain and gears working effectively
Good clothing:
- No flapping pants legs
- Snug fit around saddle
- Visible top, including reflective band(s) at night
- Appropriate gloves
Last and least, I usually wear an attorney repellant (aka helmet).