S.F.
Are you watching her sugar and caffine? She may be susciptable(sp?) to the effects and that could be keeping her up. A little goes along way in small children, so the smallest bit of caffine in the evening may be keeping her up.
Also, try and find out what HER schedule is and adjust her bedtime from those cues. Let her go through a day or 2 and try to take some cues from her. When does she normally get tired on her own? Does she lay herself down for a nap? When she takes a nap, how long is she sleeping? If she can go through a day without a nap and not melt down in the early evening, she may no longer need a nap.