I've just posted quite a long reply in the thread above this one (http://elss.qmlanguagecentre.on-rev.com/forum2/index.php?topic=24.0
) also about summarising.
All I can really offer is this:
One way to summarise a long topic is to write down (or highlight) all of the key points from your sources (whether the sources are other essays and books, or whether it's your own writing). You then organise these ideas into a logical and clear sequence, and rewrite them as concisely as you can.
It's a difficult skill, but one that you can learn through repeated practice.