For the last few months, I've been extremely busy. I've got a full teaching schedule, I'm a graduate student in mathematics, I run my own business, and I do freelance writing. My day usually starts at 7AM and ends at 11PM. Being able to schedule my day is really important and I use a variety of high-tech and low-tech solutions to get me through the week.
Any task that I need to accomplish, I'll usually write it down somewhere. I've got a stack of papers from my daily Moleskine calendar that I use for this. Making lists is a really good way to do so. You can also send yourself emails. Since I use Inbox Zero with Gmail, these tasks will get done very quickly. Google Calendar is also a great tool. If your schedule is pretty loaded, then using Google Calendar will makes things clearer for you.
When you are busy, you need naps in order to get through the day. I like to take daily naps, which vary from 20 minutes to an hour, depending on how much time I have. This usually happens during my lunch break. It's a perfect way to recharge your batteries. After a nap, I like drinking a cup of coffee to fully wake me up.
If you are very busy, the trick is to find some time to relax, no matter what. For example, I tend to sleep in late on the weekends and I usually don't do much on Saturdays. It's the day that I recuperate from the week, taking a long afternoon nap and taking it easy. It gets me ready for the week to come.