Here is a simple trailing stop robot for MetaTrader 4. I use it when I trade manually. It automatically adjusts the stop-loss when a trade reaches so many pips in profit.
There's also two other options - ForceStopLoss to set a stop-loss in case I open an order with none (also useful for brokers which don't allow a stop-loss to be set on market order open), and BreakEven, which moves the stop-loss to the open price when your trade reaches the break-even point.
The "ForceStopLoss" and "BreakEven" options don't interfere with the Trailing Stop function.
TrailingStop and StopLoss are set in pips. The default is 20 pips for each.
If you're not sure how to install robots or "Expert Advisors" in MetaTrader 4, this link has sample instructions. Ignore the part about DLL file installation. Also, you'll be installing an .mq4 file instead of an .ex4.
Update!
There was something funky going on with the ForceStopLoss feature where the stop-loss wasn't set accurately. I've changed the code and hopefully fixed the problem. Update available here. This problem didn't affect the trailing stop code in any way.