Sell all your old stuff on ebay, i.e. books, cds, dvds, any decent clothing, etc. Do bundles for low value items, i.e. 4 or 5 books for £7.50. DVDs tend to go for decent money if relatively modern and current, especially box sets. Children's clothes and boxed toys/games too, especially coming up to Christmas. Children's collector cards, figurines etc (pokemon, moshi monsters etc still sell well in job lots and for rare items), we even sold Morrisons/Sainsbury kiddie's collector cards in job lots.
Don't forget obscure things either, like phone/laptop chargers, laptop batteries, old laptops, etc - plenty of people looking for second hand things like that to keep their phones/laptops working (i.e. people buy old non working laptops to swap the keyboard, hard drives, dvd drives, etc into their own to keep them going).
We've also sold an old electric lawnmower battery that was very sought after. We did it collection only because of how hard it is to post batteries, but had people from all other the country wanting to buy it and virtually pleading with us to post it. There wasn't a single alternative available to buy anywhere and the lawnmower was only about 10 years old. People were wanting replacement batteries to keep their, otherwise perfectly good, lawnmower in use rather than ditching and buying new!
Same with old furniture you don't want to take with you. Lots of people are eager to buy stuff that still has life in it, including book-cases, sofas, dining tables/chairs. We put that kind of thing on ebay for collection only, and also on another site called "preloved" which we've had a lot of success with.