Trailing stop allows you to automatically protect the profits with your positions. It adjusts itself according to the current market rate and the amount of pips you give it to trail behind. Trailing stop is a great tool for the conservative and long term traders as it easily creates protective "airbag" for trades. There are two basic ways to set the trailing stop in your MetaTrader 4 platform - use the built-in tool and attach a special EA that will apply a single trailing stop to all orders. But before going into describing these ways I'd like to tell you how the correct trailing stop should work:

Currency Strength EA
This EA opens Buy or Sell trades in strongest or weakest pair. It cannot be backtested. Kindly use it on Demo account first & use it on your own risk.
Trade Multiplier
This expert will copy a trade with the preset volume on the same account, can be used with a signal. No cross-account functionality.
ChartEvent Test Example
Test ChartEvent is an example demonstrating the various types of ChartEvent: keyboard keys events, mouse click & move events, custom events. The source code works both in MetaTrader 4 and MetaTrader 5.
Tipu MACD
Tipu MACD is one the popular indicators in the Market. The original code for Tipu MACD is modified by removing compatibility with Tipu Panel. This version of Tipu MACD is open for everyone who is interested in developing an Expert Advisor.