Whatever you do DON'T use washing up liquid.
it contains degreasers and detergents that will strip your car of all the protection and therefore dirt and contaminants from road traffic will eat at the paintwork and cause it to dull.
Obviously some people really aren't bothered either way but if you're anything like me you will be.
The average cheap ones that are OK are things like Turtlewax etc. I'd say they're defo worth spending the money on especially if you don't wax your car every few months.