Get yourself properly organized. Before you even start the experiment, know how you are going to record your data. Obtain any data sheets you're using ready and have them with you at all times while conducting the experiment.
Write legibly. If you get sloppy, you might not be able to read your data later, meaning that no matter how good the original information was when you recorded it, the data will no longer be useful if you can't read it.
Take notes early and often. In many chemistry experiments, your results can change quickly. Reactions can start right away, take a long time to occur or change in an instant. This means you need to be diligent in your note taking.
Keep track of what you do during the experiment. Not only do you need to keep track of what's happening, but you also need to write down what you're doing to make it happen. The slightest difference in an amount of an added chemical, for example, might be the key to getting the results you want.