User impersonation using Filters/Cookies

Building an application in Flex/Java that requires a super user to be able to impersonate another user. We did this by having a spring controller write a cookie with target user's id. Then app reloads. A filter is on the app that looks for cookie and build a principal off that user id if it finds the cookie, or it builds a prinicipal based on Cleartrust headers. It then sticks Principal on session and in a thread local so java service layer can find it. However we are seeing that as the app reloads, it starts with the intended impersonated user id, but then it changes over to the super user's id. I was wondering if there were some ideas better than the cookie/filter way to maybe implement this feature.