In the context of a general trading strategy, it is best to trade with trends. If the general trend of the FX market is moving up, you should be cautious and attentive in regards to taking any positions that may rely on the trend moving in the completely opposite direction. A trend can also apply to interest rates, equities, and different yields - and any other market that can be characterised by a movement in volume or price.
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)