Transaction costs are just a part of forex trading. Either you pay for the forex spread or you pay through commission. More reliable brokers are better so sometimes you have to sacrifice low transaction costs.
Security is the most important characteristic of a broker. When dealing with large amounts of money, you aren’t just going to simply hand it over to someone that claims they are reliable. Regulatory agencies from around the world separate the fraudulent from the trustworthy brokers.
Depositing or withdrawing your funds or earnings should not be a difficult procedure at all. Withdrawal process should be smooth and speedy.
Your broker should fill your order at the market price you see when you click buy or sell. At least this is the case under normal market conditions. If you are a scalper, it is even more important to have orders executed with lightening speed.
Stable and user-friendly trading platforms are a must. Most activity happens through a broker's trading platform. Also find out if it offers charting tools and free news feed.
Problems sometimes come up, and you need your broker to be there for you at all times. Check to see if your broker has a 24-hour live support. This is just as important as how fast your broker executes orders. Some brokers will be friendly helping you set up an account, but then they seem to “disappear” when you need after sales support.