OMG, been there, done that! DD10 is in the gifted program at school, and we're still scratching our heads--how can a gifted kid be so disorganized and goofy and absent-minded? But I guess it's just different priorities!
A couple things that work in our house: DS8 doesn't get any TV, computer, or Play Station until homework is done. He does his HW IMMEDIATELY after school (his choice, not mine, but it works!). If he, say, forgets his spelling words, then no TV, etc. until the next day. That cured him of his forgetfulness.
For DD the daily stuff isn't too bad, but the longer term projects can be a problem--some nights she has a lot going on. Generally, I discuss this with her, and gently remind her on a "quieter" day that it would be a good time to work on X because you can't tomorrow night.
If my kids completely miss something, I let them take the hit. Sometimes it's the best cure for forgetfulness--fear and pain are great motivators. It's very difficult, as a parent, to do this, but it generally only happens once per child. Last year, DD got a 57 on a social studies test. She was sick that day, had the make-up during recess, wanted to be out with her friends and rushed--I can see how this happened. Well, she was in tears telling me about it--no further punishment needed, IMHO. She ended up with a B+ for the quarter and still pulled off an A for the year, so clearly, no harm done. You can bet she won't forget it, though.
and I did just fine. It's a different story now that I'm the mother though!! 
