One of the first things you need to do is actively decide how much effort you are going to put in. If you truly want to achieve a 80 or 90 mark in a test, tell yourself that you will do that, and you will tackle anything that stands between you and that mark. If you don't want to put as much effort in, that's also acceptable. It's still respectable to go through life just cruising and not always giving 110%, believe me this is an option, many people do this. So figure out exactly how much effort you want to put in, but whatever you do, don't decide to put in 110% when you know you aren't actually going to. Don't lie to yourself.
Once you are motivated to do your work, a good way to support your motivation is by use of good goal setting.
What is a goal? A typical goal in life might be to make it home in time for dinner. These types of goals are often broken, and therefore the value they hold is lessened. When I study, or when I want to do anything in life, I create what I like to call "super goals". Super goals are always very specific, are extremely achievable and are never broken. Seeing as I have not broken a super goal I have made in the past, they hold great value to me. I know that if I were to ever break a super goal, that value would be ultimately destroyed. Just this afternoon, my super goal was to have my maths homework finished by 7pm, and I got it done, because I made it a super goal.