What are the tricks behind an American accent? I find that idea fascinating.
I know I have heard from Brits that it is easy for them to do an American accent if they speak nasally, or if they have a wad of cotton in their mouth :-)
that is a great YouTube channel that can help you to improve your pronunciation. Check it out.
There isn't really such a thing as a neutral accent, exactly, but RP (Received Pronunciation British English) and GA (General American) are probably a couple of the most widely understood. The Californian accent isn't neutral, but most people in California don't use it (and, in fact, are frequently not from California), they use General American.