Should it be 10 US$ or US$ 10?

If you're going to use a symbol rather than spell out the currency's name, you should always put the currency symbol directly to the left of the digits: "$10" and never "10 $".

As for the placement of the country indicator, it's generally before the symbol/amount string: "US $10", although I have seen "$10 US" in magazines as well, so (as long as you're consistent) I wouldn't worry too much.

If you choose "USD", it seems to be your choice whether to put it to the left or the right: "USD 10" or "10 USD".

However, if you're talking about larger amounts - millions, billions, trillions - the rules are a bit more strict: "US $10 billion", "USD 10 billion"

In newspaper and magazine usage recently, it's almost always simply "$" or "dollar", with no national specifier - I looked at today's Wall Street Journal, New York Times, Times of London, Le Monde, Le Figaro, and Der Spiegel - none of them specify the country when referring to dollar amounts (although the French refuse to use a symbol or abbreviation.)

If you are trying to make it clear that these are United States dollars (rather than the mighty Canadian version)

"$10 US" or in a more formal economics text possibly, "10 USD"

It's probably best to either use the appropriate sigil (e.g. $) or the ISO currency code (e.g. USD) as a prefix, rather than mixing them. Alternatively, use the sigil, and follow the quantity with a country specifier (e.g. $10 US, $14 CA or $14 Canadian).

I work in a law office and this is how we are doing it now (lately) (US)$10,000.00!