It really depends on the class. Being in school long enough hones your skill of creating a study strategy perfectly suited to each class. When I was a freshie, i didn't know how to do this, and as a result had to work much harder than i do now as a junior. My classes are harder, but i put in less work because I have honed my ability to study perfectly for each particular class. Some would consider my strategy as doing the "bare minimum" to get the A, but I would say that unless you plan on getting a letter from the prof, you are hurting yourself if you do anything more than the bare minumum to get the A.
Common mistakes:
you have 4 finals, lets say Ochem, Physics, and two easy ones.
Say you only need like a 60% on the physics to get the A, and everything else you have to watch out for. A mistake here would be putting equal time into every class. Skimp on the physics, giving more time for the classes you don't have in the bag.
Getting into too much detail in the sciences: Say for Ochem you have a test on rections of alcohols. You wan't to make some alcohol from some ketone. You figure, hey, i could reduce that bad boy with NaBH4 or LiAlH4. For a simple problem like this, you don't really need to have the mechanism for ketone/aldehyde reduction memorized, you just need to know the consequences of such a reduction and the conditions under which it is going to happen. So for NaBH4 you want to use a solvent such as methanol, while if you use LiAlH4 you need an ether solvent, followed by protonation of the alkoxide. This is just one (poor) example of what i mean, but the point is, you need to be able to identify essential and non-essential information. This will vary depending on the prof, but nonetheless, identify what the guy with the PhD finds most important and fuggettabbout erthing else.
You can either work your ass off and get the A, or work moderately hard and get the A. Call me lazy, but i have more time to you know... have a life and all 😉