I don't know how one forecasts the value of currencies - but the Bank of New York has done so. It comes with this caveat: "The following currency forecasts are provided on a quarter-end basis and are updated monthly. Forecasts are for informational purposes only and should not be viewed as investment advice. Major currency pairs are forecast based upon economic and capital flow fundamentals, while cross-rates are synthetically derived from the majors. Forecasts for emerging markets currencies incorporate currency regime parameters including purchase power parity and linked nominal depreciation."
BTW, if you're looking for current or historical currency trends, try XE.com or OANDA, although Google and Yahoo do much the same.