Have the discipline to stick to the rules, and avoid overriding any entry/exit signals you get.
Keep a log detailing any trading errors or mistakes that you make, and quantify the effect of them. Once you have done that, then you should resolve to try and eliminate those errors. A lot of traders try and use a method which should allow them to cut their losses and let their profits run, but they found themselves unable to follow their own rules. Often they will second-guess a signal, or will try and predict about what may or may not happen. Overriding your rules may well destroy the positive expectancy you should be able to achieve by turning a profitable method into a losing one. Keeping such a log will enable you to clearly identify these issues, as well as quantify any 'profit leakage' in terms of lost R or percentage returns.
Resolve to avoid risking too much of your hard-earned equity on a single trade.
Despite what I said above, trading mistakes can happen. Also, when you open a position, you will never know whether it will be a winning or losing trade. Typically trend followers have a win rate of less than 50%. The easiest way to keep your losses small is to risk a small amount on each trade. Most traders who have been successful over a long period of time risk only 1 or 2% of their equity on each position.
Make sure you calculate your position size and initial stop placement correctly.
There are plenty of traders who use incorrect stop placement, primarily because they want to have a certain position size on a trade, without exceeding their risk per trade limit. However, this could mean that your initial stop may be too close to the current price.
As part of your position sizing you should factor in the possibility of price slippage or an overnight gap going against you. This can be a critical element should you be trading stocks in particular, where earnings releases can cause significant price gaps from one day to the next.
I use a volatility based initial stop placement, based on an Average True Range measurement. This would generally mean that, the more volatile price action gets, the smaller my position size. (Read more here on how I use volatility as part of my stock selection process.)
You will note that these three points have no bearing on your chosen method or timeframe - in other words, they can be applied to all styles of trading. It can be argued that how you determine your entry and exit parameters is the least important element of any sustained trading success you may achieve.
If you can follow these three points, then potentially you are all set up to be profitable over the long-term.