Drink lots of water and eat as well as possible, food is your fuel. Some women swear by yoga, and although it may see extreme, a pre-natal acupuncturist may really be able to help (make sure you get credentials and that he/she have studied to specifically work on pregnant women.) it may also be that you aren't getting good sleep at night because of discomforts, so maybe try to go to bed earlier or sleep in if you can. However your blood volume has doubled and the baby needs a lot from you, so some women are just tired and run down all day no matter what they do, remember its a phase, and although it may get worse before it gets better, it will pass eventually. You could think of it as a sneak peak of life after baby, those days can be very tiredsome ;)
maybe make sure you're getting enough iron? Im in my 3rd trimester and was feeling really fatigued until I found out I wasn't getting enough iron. Also, I know that caffeine is frowned upon, but I had really bad fatigue in my first and mid way through my second trimester. I worked two jobs and was a full time student, so I didn't have any time for naps. The best solution I found was drinking one cup of decaf coffee or regular black tea. It's a lot less caffeine than regular coffee, but it can still give you a healthy boost