In my time, I have in South London and Bournemouth sold many many cars and have taken tips from the professionals themselves as well as developing my own way of making virtually any car gleam.
Now, many cars out there are past their prime, why bother, its an old scrapper, people will say but I have always taken these old scrappers and as long as the top paint is pretty much undamaged, can breathe new life into the saddest of vehicles and it does reflect in the price of your sales, if it looks better than before, if it looks halfway decent, then the price is always higher as a result when you sell.
There are two ways at looking at this, when you have a newish car, people expect it to be newish inside and out and you move accordingly to that, however....
If you have an older car, then expectations change because a gleaming engine and gearbox on a 150,000 mile Sierra, many will be suspicious and expect the engine to leak like a sieve so it is best to act accordingly to that as well.
All my vehicles, bought and sold end up leaving my ownership if I am lucky with paintwork looking like wet glass and boy does that make a difference when people buy.
Lets wash the car then...
Now this may sound crazy but the first thing you can do, to help you is to take your car down the jet wash and give the wheel arches, the lower portion of the car a damned good blasting with the lance or if you have a lance attachment for your hose, get rid of the built up mud and grime in the arches, grilles, under the sills and rear underhang.