I don't have a child yet but am young enough to still be in education (Masters) and can vividly remember revising for GCSEs in 2001, A levels in 2003 and Uni Finals in 2007.
Bribery might work for some, but think of how much pressure that puts the teen under, when they are already crapping themselves about how you'll react when they get their results? I know that personally, I was disappointing to my parents if I didn't get As (my mum said to me when I got AABBB for my A levels that I should have got 5 As and that it was down to too much partying). So add to that the monetary factor and I would have been a nervous wreck.
I appreciate all teens are different and thankfully I was motivated enough to revise. The key for me was to do a revision timetable. 8 subjects or topics a day, in 45 minute chunks. After each 45 mins were up, I would get half hour break to sunbathe/MSN (pre-facebook days)/watch MTV/eat lunch then start again. I also used to do 3 45 mins in the morning, then 3 in the afternoon, then 4 in the evening.
Having a timetable which allowed me some 'me' time but also ticked off all the subjects was extremely motivating (but then again, i'm an organisation freak and am now paid a lot of money to organise other people!)
It also helps if you have somewhere dedicated to do study, i.e. not just the sofa/bed, but a desk with good light, plenty of peace and quiet and not too many distractions. Dining room table is good if desk in bedroom is not possible.
If it does all go wrong however, just remember with enough commitment and drive, good things can come out of bad grades. My dad left school with 3 O levels and worked his way up from shop floor of a major financial institution to Director level. And his teachers told him he would only ever be able to be a mechanic. (Not that mechanics are rubbish, but they didnt think he'd make it in the corporate world).