T.A.
We moved to a big boy bed at 15 months. My son did great for the first month. We never had a problem with him getting out of bed at night or for naps. But one day he realized it was light enough in his room to play and mom would never know as I was out of sight. I just moved him to a pack n play, for naps. That worked for a week until he learned to crawl out of that. What I finally had to do was put him down for his naps and sit by his bedroom door with it cracked. The second his foot hit the floor I walked into his room said, "No Nap time. If you get out of bed again you will get a spanking." This was by far the hardest thing I have ever had to do as a new mom. It took 2 hours of me sitting by the door and going into his room, however he gave in and finally took a nap. Everything was great for a few days, and then he tried it again. This time it took 10 minutes and he gave up. He would test me a few times and sit at the end of the bed and get his foot almost to the floor and then pull it up again. I thought that was pretty funny, cute and smart. Now at 18 months I have absolutely no problems at all with him getting out of bed. Whatever method works best for you stick to it and don't give up.