The cheapest method I have found and one of the best to is use Teak Oil, really cheap and had some AMAZING results from it! Used it around 3 months ago on a bumper, did two applications and the bumper still looks like new...

I got the tip by asking a customer when he dropped his car off if he had new bumpers on his car as they looked MINT (Mk2 Golf), and he said no, just used teak oil lol

Gave it a shot and it really is VERY VERY good!

These days I usually apply it as below

Usual wash routine
Coat bumper/plastics in teak oil
Buff off
Coat again

Then when you wash the car again, a single top up layer and it lasts ages

Im a die hard GTechniq fan to but the cost of the product vs its coverage meant its on the expensive side, teak oil I found 1ltr for £2.50 and so far we have done approx 8 cars with it and only used 1/4 of the bottle yet!