Most babies fall into one of three categories in terms of how they like to fall asleep. This was certainly true for my two kids! Here are the three categories:
Suckers: They fall asleep easily with a pacifier, thumb, bottle, or nipple.
Motion junkies: They fall asleep easily while moving such as in a stroller, car, swing, or while being rocked.
Cuddlers: They like being held to sleep and prefer human contact.
Understanding how your baby prefers to fall asleep is helpful, especially for naps. Naps are more difficult for children (and everyone) because of a less strong sleep drive during the day. You can use this to your advantage for those hard to get but needed naps, especially when you are not at home or not in your normal routine! Independent sleep (without any of the above options) is really important for nighttime but less important for naps. Look here for more on independent sleep: The Single Most Important Sleep Training Tip.
My daughter is a motion junkie. She fell asleep quite easily in the car or stroller. I uses to plan naps around car trips or stroller walks, especially on the weekends. She is five and still falls asleep sometimes in the car. My son is a sucker. He still sucks his thumb at age 3 and loved the pacifier, though we took it away awhile ago. His favorite candy is suckers and he loves drinks with straws. Which type of baby or child do you have? Let me know in the comments.