Requesting advice for stay in London

Let's take it as a given that I should have sorted this out two months ago. That said, I'd be grateful for suggestions about finding accommodation in London (England). We plan to be there for a week at the end of August. We don't need anything fancy, and it does not have to be in central London so long as it's near some sort of transit. Any particular neighbourhoods I should look for, or avoid? Any other suggestions for a first-time visitor? oursin, cosmolinguist, got any advice for a clueless Canadian?