If you could truely predict them, then you could be a very rich person very quickly. Currency is traded just like stocks - if you always knew when it was going up or down, you would make money with every transaction.
Since currency is freely traded prices can fluctuate with no conceivable reason. Some large international company might decide tomarrow to switch most of their money from dollars to euros and then the price might change significantly due to supply and demand. However, in the long run, it generally follows the economy of the respective regions. If inflation started running rampant in America and not in Europe, then people will value the dollar less than the euro and the price will change. Recently the American economy is becoming less trusted and the dollar price has dropped significantly.
Unless you are exchanging a lot of money, it might be wiser to search for the lowest cost of exchange instead of guessing about the direction of the exchange rate. For example if I go to my bank in my home town and switch $1000 to Euros I would have to pay them a $50 fee and then use a terrible current exchange rate. However I have other options. 1) I can go to Europe and just use my creditcard and I get a great exchange rate with no fee. 2) I can go to Europe and use my ATM from the same bank that wanted to charge me $50 and instead only pay a $1 foreign ATM fee plus I get the best exchange rate possible. Conclusion: the location of your exchange and the fees they charge might make a much larger difference than an unpredictable change in the exchange rate.