S.T.
i agree with the previous posting. it's time for a big girl bed - my daughter went into a twin bed at 24 months, and though it's a transition, she loves her big girl bed. also, i would try to see if your daughter is getting enough physical activity in the morning hours to wear herself out a bit. days when we're not as active, my daughter isn't as tired, and will sometimes fight her nap. i try to be sure we get outside for a walk or run around, dance inside to music for a while, roll around and tickle, or chase each other around the house... something to get her a bit more tired.
fighting naps is one way to exercise independence!!! they NEED naps at this age still - and they need the sleep to grow. i have a gate on my daughter's door, but when she gets out of her bed, i SILENTLY walk into her room, pick her up and put her back in bed. i am sure to not say a word, because as soon as i do, my DD thinks that there is room for negotiation - there is not. she knows from us talking about what is expected at nap time that she is to stay in her bed. she gets a book and her stuffed animal in her bed at nap time too, so she can quietly sit and look at her book, but can NOT get out of bed. there are days i don't have to go in at all, and other days i have to go in there 10 times! just depends.
when my daughter gets overtired, she fights sleep even more and actually sleeps less - she wakes in the night, sometimes has night terrors, etc., it's a vicious cycle. so we really try to stay on her schedule as much as possible. life gets in the way sometimes, and if she misses a nap or two, there is a price to pay.
believe me, nothing is perfect. there are days when she just won't fall asleep. i still expect her to stay on her bed and this can last about an hour or hour/half - she is looking at books and resting, but not sleeping, so i am sure to adjust her bedtime that night to have her go to bed anywhere from 30-45 minutes earlier than normal. she needs the sleep!
hope this helps! oh- one thing we really were happy about was the fact that we bought my daughter a day bed (twin bed), so with the guard up on the one open side, it's like a huge crib... we think this really helped in the transition to a big girl bed, especially since we did it when she was younger (24 months). something to think about as you shop for a new bed for your daughter, if that's what you decide to do. good luck to you... :)