I think if you have fun, hands on activities that will keep them engaged is always a good way to keep them motivated. And try to motivate them by having rewards for them also, like extra time at centers or recess. Give them something to look forward to after doing their work!
I think choice it a big one. I have two students that will to do nothing! It can be very frustrating because they will sit in their desk with the attitude, "Make me." What I have tried doing is allowing more choice and a reward base system. I personally am not a fan of rewarding students. I believe that not everything entitles an award. So in the morning the students are to complete their morning routine which is story writing. On the other hand, my two struggling students I offer them three choices. I say, " You can either read a book, complete the morning or go on the computer. If you complete this you'll get extra snack time, recess, computer time, etc"
Also what I found was to talk with those students first thing in the morning by asking them, "How was your morning? What time did you go to bed? Did you sleep in again?" By asking these questions the student will open up and let you know that they are either in a bad mood or in a good mood. Then ask them, "Ok so you said that you slept in and missed the bus which got your mom upset. How can I help you this morning?" They may give out suggestions or you may suggest something.
This is what I found that works. Hopefully it helps you.
Show them you care ... get tough. Send weekly progress reports home, make e-mail contact with the parents and bring them up to speed ... don't wait! Call home, invite the parents in, turn the tables ... have the student take "ownership" for their actions, learning or lack of learning. If the student fails or is retained ... you made the extra effort to help and provide numerous opportunities to succeed.
I think every student has motivation to learn something, it's just not always aligned up with curriculum or how we teach. Find out what it is and see if you can incorporate it into your activities. I have a couple of kids who love graphic novels. I plan to have a few choices of how students will represent what they know about forces after our science unit. I'll include comic life as an option. I also agree that you need to hold students to high standards and follow through, as David commented. Good luck!
The best motivation that I have found is humor. When I used to teach and students were getting off task or bored, I always tried to have a good laugh with them once in the morning and once in the afternoon. It was AMAZING how that would snap them it out of the doldrums.
I find a combination of several of the methods listed by others works for me. Here are some specific examples:
I'm not sure what grade you teach, but starting students on Edmodo is a great way to engage them. The site is a social networking site, similar to Facebook, so students are extremely engaged. However, it's a closed network that gives the teacher total control. For more information on using Edmodo effectively, check out this blog post. (A side note: because students love Edmodo, it's something that can be given or taken away, based on behavior in other tasks.)
When it comes to choice, there's not a ton you can do -- there are specific standards and objectives that students must learn. But you can give them some choice in how they learn. Specifically, I allow students to choose how they'll present their learning to the class. For a class presentation, they can create a poster, a short film, a comic strip, a glogster, or a Prezi. They still need to communicate the same objectives, but they can choose the medium that best suits them. For more information, check out this Prezi.