The field of optimizing study or practice time is quite interesting. There are elements of wisdom from diverse points of view, such as from athletic trainers & coaches, elementary and high school teachers, musicians, and educational psychologists.
Here are a few ideas:
1) make a commitment to spend regular, frequent periods of time in study or practice
2) make your study or practice time interesting or fun
3) if your attention is failing, try to compete with yourself gently (e.g. put a mark on your page if you catch your attention wandering off); but also allow yourself brief breaks. In order to control this process (and to prevent your brief break from becoming a 6-hour break), you could use a timer. During breaks, you could rest quietly or go for a walk, perhaps reviewing in your mind some of what you have just learned. During periods of decreased attention, you may need to allow for more frequent breaks.
4) frequent review helps with memory consolidation. If you have just learned something, go back right away to remind yourself of it--maybe ask yourself, and answer to yourself, a few questions about it, rather than immediately plowing ahead with the next chapter.
5) Sleeping after learning improves consolidation of memory. Slow-wave sleep, which tends to occur in the first few hours after you fall asleep, is particularly important for memory consolidation. In one clever 2007 study published in the presitigious journal Science, subjects were exposed to an odor when learning a task. If they were exposed to that same odor during subsequent slow-wave sleep, their retention of the learning task was significantly improved. Here's the reference:
This suggests a simple aromatherapy technique to enhance your studying: infuse your study environment with a distinct, pleasant fragrance (for example, try an aromatherapy oil) -- then infuse your pillow with the same fragrance afterwards. During an exam or test, try infusing the same fragrance on your skin or clothes (just don't overdo it, or you might irritate the people writing their exams next to you!)
Furthermore, there is evidence that brief naps (60-90 minutes) in the middle of the day can help with memory consolidation, motor learning, and can also prevent the deterioration of mental and physical performance which tends to happen in a long day. Here is one reference about this:
6) choose a study or practice environment which is psychologically pleasing. This could include multi-sensory environmental manipulation, including access to healthy foods, smells, comfortable seating, quietness, soothing background noise, etc.
7) if part of the learning task requires repetition, make special effort to infuse the repetition with something imaginative.
8) if part of the practice is for exam preparation, etc. then you could try to mimic the exam environment repeatedly--e.g. by doing mock exams at the same time of day as the scheduled exam, or by doing these practices in the same physical location as the actual exam, if possible.
9) if the practice is for a performance, it can help to record yourself periodically; when you hear or look at your recording you may need to be critical but you should also consciously affirm the aspects of your performance that went well. Self-criticisms should never be in the form of a personal attack (e.g. "I'm stupid!") but should be gentle observations of areas to work on or change.
10) a tutor could be quite helpful, not merely to "teach you" but as a motivational figure to help you practice or study more efficiently or with greater enjoyment (along the lines of a personal trainer for fitness). A friend or study partner could have this type of role, provided the friend does not become a distraction from your work.