A little tip, let it sit in the sun for about a hour or so until it feels like it is going to burn you. Then take it in is shade and let it cool off. The heating/cooling of the paint makes it stick better. I think it is because the expanding/shrinking of the paint. I also found that it is better to paint when it is cool and then let it dry when it is warm. (painting at night and then letting sit till the afternoon) This also helps with the drying..

That method also works when spraying too. I've been painting stuff for about 6 years now and that is just what I found. Good luck!