My 18 month old son has been doing one nap since about 11 months.  He goes through phases when he goes down easy, and then there are times, like today when he gives me the ultimate run for my money. 

we go through his routine, book, some milk, and rocking in the chair.  then I put his "fish" on and lay him down.  sometimes he lets me do this without a peep and falls asleep easily.  Today he cried briefly when I left but then calmed down and talked to himself for a while.  half an hour later he was crying and calling me.  But if I go up he just wants to play.  He'll ask to be rocked but then will start asking for books or milk or just starts getting squirmy and tries to get down.  Or he'll ask me to hold hid hand in the crib but then he turns it into a game.  I hate leaving him crying and I know he is not trying to manipulate me on purpose but I feel so awful!  Right now it is 1:45 and he is still awake.  He was yawning and rubbing his eyes at 11:30.  I dont get it

I would say he is fighting and I would let him be and cry it out, as long as he isn't making himself sick and throwing up when he cries. I let my daughter cry and some days she goes to sleep and some days she settles down but plays and I figure that's better than nothing at least it is quite time for both of us. Once he figures out that its not a game and he can't pick his nap time it should go easier.

The other thing is that you might try heading for bed when he starts rubbing his eyes and shows signs that he is tired, you could be waiting to long so he gets is second wind.

I know it gets tough listening to him cry but give it a try and see what happens. Do you have soft music or a fan or something in his room to mute outside sounds? That might help calm him down to.

Is he teething? I noticed that would put a damper on our schedule and how easy daughter went down.

Those are just some ideas I have. If you need anything let me know. Good luck, he should grow out of this. It could also just be a way that he is testing you to see if you will give in, stand strong.

Give him a bath in the Johnson and Johnson Nighttime Bath, or you can use the Nighttime lotion. I don't use it everyday, but it seems to work on those sleepless nights. I'm sure it would work for naps. If it seems to make him sleep for long periods of time, just use a little bit of the lotion. Herb shops have a good variety of stuff for babies and kids and they have stuff that helps them relax ( same stuff that is in the nighttime bath and lotion) but it is in a spray, you just spray it a few times in the room and it calms them down. I used to not believe in all the herb stuff, but I've seen it work on my friends kids.

Although my son is much younger (6 mths), he has been very challenging on the sleep/napping front from almost day one.  It's only in the last little while that it's gotten easier to get him to bed.  I've changed our routine so that he doesn't get rocked or nursed to sleep anymore.  Instead I lay him down when he is very drowsy but not yet asleep.  At the beginning there was some protesting, but it only lasted for a few days, and now most times he goes down with only minimal resistance.  I completely agree with everything that I've read that says if you rock or feed your child to sleep they will become dependant on that.  I also agree that the environment is key - i.e. dark, calm room and pre-bedtime lotions such as J & J Bedtime Bath products.  And definitely try to catch him when he is showing signs of tiredness - my son has been very active from birth and if I don't catch him in time, his second wind is fierce!