My top tips :
i) Make sure it doesn't smell awful. If a wave of wet dog hits you as you enter the front door it's not good - first impressions last.
ii) Get as much light in the place as possible. Pull the curtains well back and prop doors open if necessary to get light into dark rooms.
iii) Look in each room and try to pick out the thing that is most manky that attracts your attention and fix it. For example it might be the shower curtain in the bathroom, a worn door handle, a crack in the wall.
iv) Clean the kitchen and bathroom ruthlessly, especially the toilets, make sure they look as good as they possibly can.
v) Make sure the agent knows the good stuff. So for example if the boiler is new, or you are leaving appliances with hardly any wear, or the kitchen worktops are high quality make sure the agent knows that.
All the above is pointless if you don't get the price right. Recently I have seen people price aggressively through OITRO. This means they get the chance for price discovery without the risk. You can price on the low side, and the go to sealed bids if you have lots of interest. Similarly if no one turns up you can lower the price. But the key thing is getting price discovery without selling for less than you can.