including controlling more variables, improving measurement technique, increasing randomization to reduce sample bias, blinding the experiment, and adding control or placebo groups.
Controlling more variables is about making sure as few things as possible change during the experiment. An ideal experiment is where one thing is changed, and one result is looked at. Everything else should remain the same. So for example, if you wanted to know how fast balls of different masses will roll down a particular hill, you would change the mass of the ball, and keep everything else the same. You'd keep the material of the ball, the point of release, the measurement location a method, the humidity of the air, the height above sea level and anything else you can think of the same. The more you keep the same, the more likely it is that your result will be valid.