No, definitely keep pushing. I know some pre-prep schools in central London (as attended by Wills and Harry) where there are hundreds of kids on the waiting list but those parents who persist and stay in contact with the school will be kept in mind by the Admissions Director. It can't hurt.
We were on the waiting list last year for the 8+ and I was told that there were no No NO places by the admissions folks. Recently someone told me (a bit chinese whispers) that the school was asking for updates on some kids from the previous year. Would I sound a bit batty if I called them to say we would be still keen for a place?
They'll hold them open as long as you still meet the criteria for entry. The best way to get yourself bumped up the waiting list is to keep calling to ask if any places are coming up. Most schools tend not to check the data on their systems very regularly and there will be lots of people possibly on a waiting list who are no longer interested or whose children now don't fit the entry criteria.