Use as much history as possible when backtesting, allso make sure the history is good quality (don't need to be perfect). Most MT4 and MT5 brokers offer good quality history from 1999 on major symbols.
One good backtestingmethod is to split the test in two parts, one backtest and one forward test. For example like this: The backtest you run on the years 1999 to 2013 and for this period you build you strategy. When you are done and feel comfotable with your results you do a forward test with the same strategy but on the years 2014 until today. This forward test will help you determine if your strategy might work or not. Remebember: If you can't succeed in the past. You will have no chance in the future!