Control as many variables as possible. This is not counting the independent variable. A variable is a factor within an experiment that is subject to change. The independent variable is the factor that you purposely change within an experiment. For example, consider an experiment in which you will measure how long a candle burns in relation to the length of the wick. The independent variable would be the length of the wick. When you conduct the experiment, you would need to control other variables, such as wind flow and the types of candles used.
Repeat the experiment as many times as possible, using the same exact procedure and controlling the same variables. The more time you repeat the experiment and achieve the same results, the more reliable the results will be.
Take measurements using the most precise measuring tools possible. If you are measuring something fairly small, for example, a ruler that shows millimeters would be more accurate than one that only shows inches.
Repeat your measurements. By measuring something multiple times, you are more likely to achieve accurate results.
Write your procedure with as much detail as possible. This will help others to be able to duplicate your experiment, which is essential in order to prove that it is scientifically sound.