Read a lot. No matter what the subject is, anyone who reads a lot is guaranteed to memorize the material and know it by heart. In science, it is important to know a lot of formulas and theories. The best chemists are able to make their concoctions off the tops of their heads. Besides, not all scientists have room to place their notes in front of them.
If you do not like to read, you can become accustomed to do so, no matter how easily distracted you get. Start by reading a few pages every evening before you go to bed. Then, gradually increase that amount and the number of times you read each day. Practice reading with a partner or say the words out loud.
Students are encouraged to read their textbooks and lesson plans before they go to class. When you become familiar with the old content, you have more room to squeeze in the new content. Reading the old material, followed by the new, is the best way to keep up with a fast-paced class.