We just got my 15 month old to go down for a nap peacefully. We always do the same thing before each nap and before bed too (minus his night bath). We found that we purposely started getting him to fall asleep in the car, stroller, where ever it was easier - we wanted to change this so here's what we did:
You might want to decide first if you mind walking/rocking him to sleep or if you want this to change??
What we do:
-10-20 min. of only quiet play before nap time (books/puzzles...quiet show)
-small snack, drink of milk
-diaper change - the whole time we remind him that it is going to be nap time soon
-go up to room, 2 more books in room
-make the room dark
-say night night to the cat/toys etc.
-put him in his crib, turn on his aquarium music in his crib and we leave the room
-we only go back in if he needs us (hard cry) and we try to give him 10 sec. to settle himself - This at first took us 1 hour but now 5 min.
-like I said, we only go in if he needs us and then we just lay him down, leave and say night night
We found that always keeping with the same routine worked for us
Good luck