Before I sold mine, I decluttered, removed all personal belongings / knick-knacks etc and gave the walls a fresh coat of neutral-coloured paint. If you just keep things pretty simple, it will appeal to a far greater range of buyers.
I sold mine with all contents included, which seemed to be a big bonus for my buyer.
Regarding price, well the auctioneers know their business and were initially guiding it at what I thought was far too high a guide price, but to my great delight, it went into a bidding war and I got well above that price.