our guys tried to get out of the portacrib, but were never successful. but when i put one of them into the portacrib, i stayed right there and said gently (or sternly if they kept trying) 'it is time for lying down and sleeping, if you use your good manners and go to sleep tomorrow you can try sleeping in your big (boy/girl) bed tomorrow. beds are for laying down and sleeping'
then i would leave, and if i heard them stand up i would go back in and do it all over again. the key for us was consistency - because we really stuck with it it only took a few days. we didn't change tactics and honestly, dd was only back in the pc once, ds took about 4 days to realize what he was missing.
as to rewards, my experience at this age is that it has to be instantaeous, so a sticker now. stickes here are only good if they can put tem on their clothes or on the backs of their hands - a chart is nothing to them.