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first, create a portfolio of different uncorrelated trading systems on one symbol;

then, you can gradually create a set of portfolios on different instruments.

A portfolio of simple trading systems will be more resistant to market changes than one complex system with multiple optimizable input parameters.

Important aspects of choosing a developer

So, you want to implement your trading system in the form of an Expert Advisor: you have created an order in Freelance and received applications from different developers. How to choose the optimal Developer in terms of cost and quality?

An experienced developer will not tell you about the complexity of previously developed systems or trading system variations, but will ask questions regarding your Requirements Specification. In other words, he will not try to impress you. Professionals appreciate time, so they usually do not waste it on philosophical discussions about the nature of trading or difficulties of programming.

The developer may request more details, in addition to the provided short description. If the order is generally clear, the programmer will provide information on the order cost and time.

A responsible Developer will point to unclear points in your Requirements Specification. If your order lacks detail, you can clarify them later with the programmer, as well as pay for consulting services, increasing the time and cost of the order.

A good programmer appreciates time, so he will try to clarify unclear points in order to start working with a clearly and thoroughly prepared Requirements Specification.

What the programmer cannot do for you

It may happen that a trading robot based on your system will show losses during testing, although you checked it manually and were sure that the strategy would be profitable. As a rule, the reason is that the Customer was able to check the system manually using a short time interval. The Strategy Tester allows getting trading results for any available historic interval. What can be done in this case? A programmer cannot make a profitable strategy out of a losing one, but he can suggest some ideas for improving the quality of entries. For example, you may add some trend, volume or other filters to avoid false signals.

Also, your strategy may work better with other parameters which may suit better for given market condition. Optimize the EA to study dependence of optimal values on the year, volatility and other factors. This will help you identify weaknesses or limitations of your system. But you will have to do it yourself, because the Developer is only a code programmer, but not a trading system analyst.

The last part concerns programmer's errors. It is almost impossible to develop a program without errors. These may include code errors, when a wrong code is written for a correct algorithm, as well as logical errors. In both cases, you will have to find them yourself.

You should understand trade server return codes, as well as runtime errors.

Make sure to indicate in your Requirements Specification the need to process the results of each important operation and to log error codes. You may additionally enumerate such important operations: sending of trade requests, calculation of Stop Loss and other operations.

Such messages should use predefined macro substitutions, which will help you to find the reason and exact location of the wrong behavior.

These three rules will help you analyze the situation and communicate with the Developer.