1. Get enough sleep.
Sleep deprivation is most common cause of low energy and fatigue. Most adults need 8 hours of sleep every night. If you are sleeping in late on weekends, it's a sign that you don't get enough sleep during week.
If you have to wake up early in morning, go to sleep earlier in evening. Going to bed even 30 minutes earlier can make a huge difference in your energy levels during day!
2. Reduce clutter.
Not being able to find anything among out-of-control piles of papers is stressful. Clothes that you never wear, papers that are falling everywhere and useless items that you feel sorry to throw out can invade your space and take over your life.
Clutter causes stress and saps away energy. Dump all unnecessary stuff in a big garbage can, and you'll be able to breathe easier and have more energy.
3. Drink more water.
Being dehydrated makes you feel tired and irritable. If you are not thirsty, that does not mean that you drink enough water. By time you feel thirsty it's too late - you are already dehydrated.
Most people need at least 8 cups of water per day. The easiest way to know that you are drinking enough is to buy one of those "8 glass" water-bottles that they sell in most drugstores.