The first thing I would do is clear out the attic. Anything that you don't want to take with you to your new home should be freecycled/taken to a charity shop/binned. Be ruthless - remember you will pay more to the removal company the more stuff you have!
Hopefully that should free up some space up there so that you can declutter and then store stuff up there (obv only stuff you want to take with you...)
When it comes close to selling time, start packing away photos, etc if you have lots on display. (I once viewed a house where there were "glamour" photos of the wife in the bedroom - yeuch)
Anything will sell at the right price, my friend is a negotiator and she said the only things selling are the realistic sellers who have decluttered, have done all the little jobs that need doing but don't think that their house is the only one in the world that can still command 2007 prices. People can't see beyond the end of their noses, if something is laid out as a nursery and they haven't got kids they won't buy it, same with children's bedrooms people seem to think they can't or shouldn't have to decorate. Don't spend too much on anything and think you can add it to the asking price, you can't those days are over for now.