I find that the forums at Detailing World are fantastic for this sort of thing (e.g. they have a specific sub-forum on cleaning interiors). Beware, if you start checking it out though, you'll probably end up spending loads of money on detailing products like I did !
They've got lots of handy guides over there, e.g. this one on basic good washing practice.
A clay bar is something that you can use to remove impurities from your paintwork, things like metallic particles that will have settled in there over time (e.g. industrial fallout), general grit etc. An example is Bilt-Hamber's Auto-clay, which has the advantage compared to most clay bars that you can use water as a lubricant (most others need specific products as lubricant). Auto-clay soft is reputedly very easy to work with.