Hi there,
Weathering is a difficult task, as everyone has a different perception (and opinion in some cases) of what is acceptable.
I use a mixture of paints, weathering liquids and powders.
I utilise the paints for more heavy staining, but I usually work over the areas with powders too. But one of the better products (in my own opinion) are weathering liquids.
Available from Modelmates (and Humbrol I think as well), they are painted on into corners and raised detail, then I use a cotton bud dipped in water to wipe off the excess, leaving the corners and raised detail picked out more. Then usually weathering powders and finish off with a matt varnish.
Weathering liquids can be found here or at model shops http://www.modelmates.co.uk/
The main thing is practice and patience.
Regards
Jinty ;-)