The best way of demonstrating this is by looking at long-term charts of various currency pairs. Let's take the GBP/USD as an example.
If you look at the 30 year weekly chart and draw an EMA (100) to show the trend, you will notice that apart from a few periods of sideways movement, the pair is nearly always trending upwards or downwards, and often for long periods of time.
For example, between 1981 and 1985 there was a long sustained downward trend where the price went from about 2.4500 to around 1.0400.
Similarly it then rebounded and trended upwards reaching it's peak in 1992, and if we look closer towards the present time we can see that the GBP/USD has been trending upwards since 2002.
So the point I want to make is that the forex markets generally move in trends (short-term and long-term), so by identifying these trends you know you should only be trading with the trend and not fighting against it. Your decision therefore is when to enter, and not which direction you should be trading.
Also, although there are clearly defined trends, prices do not move in a straight line. You will notice that price patterns move in waves and there are always retracements along the way. You ideally want to long pairs at the bottom of a wave and sell at the top, and vice versa, but it's obviously easier said than done.
However, the best way is to use technical analysis for confirmation of a new wave upwards or downwards.
For example if there's a strong uptrend, and you see a sharp sell-off, and then a new move upwards, a good entry point would be when the price starts moving upwards again after the sell-off and technical analysis confirms the uptrend is still intact.
An example of this would be in 2005 when the GBP/USD was heavily sold off going from around 1.9500 to just over 1.7000. We can see that after crossing below the EMA (100), it crossed back above it at just under 1.8000 in 2006 and continued to rise, so this would have been a good entry point (and as it turns out a highly profitable one as well as it's continued upwards ever since).
So to sum up, the simplest way of trading is simply to identify the trend and trade in the direction of this trend because forex markets are nearly always trending upwards or downwards. This way your only decision is when to enter, and you can use technical analysis to determine this.
