A quick clean
A good valet can transform a dirty motor, and you can pay as little as 10 for an inside-and-out clean.
A decent polish and some elbow grease will also help restore the paintwork's lustre and remove surface blemishes.
Little things such as broken numberplates or missing wheel covers make a car look neglected, and replacing these won't cost too much.
You should also fit new light lenses if the old ones are cracked, and a new set of carpet mats will lift a tired interior.
All of this restoration and refurbishment will make a car look great, but we'd urge a bit of caution: you could easily spend over 1400 on repairs and there's little chance of you seeing all of that back when you sell the car.
Spending this amount of cash is only worth it if you've got a prestige or desirable car that's only a few years old. These generally end up on dealer forecourts, so you'll get a better price.
Absolutely do not roll up to a garage with a car that looks like a mobile skip. The dealer will assume (quite rightly) that you haven't taken care of it, and will lower the value as a result.