Create a timeline. While it can strike earlier, senioritis usually strikes during the last half of a student's senior year. Even if you don't feel it, ask friends who have graduated. It's coming. With that in mind, create a timeline of assignments and tests for the semester to get a better feel of the workload ahead of you. If you aren't given a syllabus at the beginning of the class, then simply ask your teacher. He or she will likely be glad to help you fill in your timeline if they know it will help you better prepare.
Enlist a partner. Whether it's a friend, family member, or trusted teacher, you want someone to help keep you on track. Part of senioritis is having a severe lack of motivation, so you'll need someone to pitch in when yours fails.
Examine the timeline, and create a second timeline for yourself, with self-imposed due dates. Concentrate most deadlines into the beginning of the term. Many assignments have end-of-the-year due dates, so instead of being inundated with work when you least want to do it, begin your work when you still have motivation left.
Concentrate during the week and play on the weekend. Your temptation to take the day off will grow stronger as the end of the year approaches, but if you can concentrate during the week, especially the beginning of the week, you'll still be able to keep up. If you also stick to your timeline, you'll be able to reward yourself by taking Fridays or Thursdays off further on into the year.
Start whatever you can. Even if it means you just have an outline for an assignment or a couple words on a page, starting an assignment is the first step to finishing it. If you begin the majority of your assignments as soon as you're handed them, you'll have more time to enjoy yourself later.
Take some time off! Burn out comes when you don't allow yourself a break.
Try to adapt your assignments and studying to your habits. If you work better alone, work alone. If you do well with groups, work in a study group. Just don't push a type of studying on yourself that doesn't work for you. If you go with what works, you'll get more done faster, and be happier doing it.
Stick to your schedule. It's the best thing you can do for yourself in terms of getting through senior year with both your grades and sanity intact.