Main thing is to try and make the house as appealing and spacious as possible.
So, all lights on, all bits and bobs off sides/shelves and away, all toys in car/garage/wardrobe. Push furniture right back so that max area of floor shows, and try and find somewhere to put odd chairs/stools so that they don't clutter.
Fresh, light and as plain as possible bedlinen, and plump all the cushions up on the sofas.
If you have patterned rugs, then take them up (to me they look like they are concealing bad floor and distract from the room)
If you have a garden, tidy up, cut the grass, and put toys away
Look at Rightmove for your area and house type, and see what jumps out at you in photos
Also, ask a good friend to come over and tell you honestly what they would do - and don't take offence, this is about selling, not personal taste. So, for instance, the mural that you lovingly painted in the downstairs loo needs a coat of Dulux magnolia one coat - else in peoples eyes, your house will be the one with 'the dreadful loo', not 'the gorgeous kitchen'. One house we viewed had a teddy with chains on in the bedroom - we only remember that, not the large garden etc