I found involving older students that weren't super-motivated to be extremely hard, as well. The ideas in "Industrial Strength WBT" saved me from a mental breakdown, I think...
Keep in mind, more / less homework will not motivate kids who don't plan on doing their homework in the first place. In fact, extra / dextra credit won't motivate kids who don't care much about their grades, either.
On the other hand...there are things that motivate them. You just have to find them. Game-time, even high schoolers will go for it because it means a few minutes less time doing 'class stuff'... talk-time (if your admin is ok with it) it only ends up being 2-3 minutes anyway...
BYO Snack reward...if they win, they are allowed to bring and eat a snack in class the next day (IF, of course, they stick to the agreement of keeping the class clean. If they break it, they lose it.)
Music time, or 'Boombox!" (see the 'Teaching Challenging Students' manual on page 107)
At any rate, observe YOUR students...and see what can motivate them! Every group has something they will work for.
The biggest difference, out of every strategy that I used, was from Leadership Training. Read about it in Chapter 23 of the manual...it was the thing I seriously doubted would work...and the thing that ended up working WONDERS. My most difficult kids turned out to be my best leaders. I was dumbfounded, but thrilled. It was nice to be glad when they walked through my door, rather than wanting to crawl in a hole...
Remember, when you try something, try it with fidelity--always give it at least a month, more for more complex things like leadership training... you will see big changes.