The Value of Past Papers
I recall chatting with a parent whose son had recently written 13+ scholarship papers to a well known senior school. She recounted that he hadn't been shown past papers by his prep. school until the week before the actual assessment. We were both astonished by this and agreed not only that it was bizarre but also that it had been detrimental to his preparation.
Whilst some schools are cagey about releasing past papers, many are only too happy to provide these (or at least sample papers) to assist prospective candidates. In the case of Common Entrance and Common Academic Scholarship past ISEB papers can be ordered from Galore Park. School-specific 13+ scholarship papers can sometimes be downloaded directly from the school websites (e.g. Eton, Radley, Harrow, Westminster, Oundle , Winchester); otherwise, they can often be requested from the respective admissions offices.
Do not underestimate the value of past papers. Practising past questions is important, naturally, but simply becoming familiar with the format, layout and "feel" of the papers is also a crucial part of the preparation process.
However, a word of caution: the number of relevant past papers is finite so do not "waste" them. Save at least two of the most recent sets to practise as mock exams against the clock. The benefit of this is lost if you have already looked through every paper you can find.
Past papers are immensely important, but be sure to use them sensibly and thoughtfully in order to derive the greatest benefit.