You take amphetamines. aka Academic steroids..
If you are taking a class because of a requirement, I found that they key is to redefine reasons for being there. Don't be there because you have to be, be there because you want to get a good mark. For example, if you want to be a Biologist, and you are forced to take an english class, envision you being a biologist, and the fact that your dream requires that you get over this 'english class hurdle'. You getting a good mark in the class you don't want to be in will help you achieve your end goal -- it helps provide a new motivation to care. Another way to do it, is self study a bit to look for other uses for the topic. Say you have to memorize all types of skulls (totally random examples being used), find out something you may like, beyond your class about skulls,maybe the fact that it is bizarre that they encase your brain, the varying sizes dependent on species, etc...then exploit that topic and try to find ways to bring back your general study to the part you are interested in. Often times we are not completely disinterested in a topic; we need to search for the part we care about. Good luck :)
think about the positive things that will happen if you pass
pretend...it works for our politicians...
lie to yourself until you are convinced that you are interested in the subject.
You can't develop an interest where none exists. So you don't study it. Unless you want to force yourself!