Go to a location with less obvious distractions. If you're attempting to concentrate in the same location where you pursue leisure activities, your brain may be subconsciously responding to that association; this association should be broken, and you should instead work in a location that doesn't naturally feel like a place to pass the time. Likewise, avoid unnecessary stimuli that can be used for procrastination; for example, turn off your computer's ability to access the Internet unless it's totally necessary (and even then, it may be an option to simply download the resources you'll require).
Notice what causes you to lose focus. Just before you get distracted, try to remember what it was that took your attention away -- for example, looking out the window. By noticing what causes your procrastination, you'll be more effective at preventing it.
Timetable yourself. Don't assume that you'll be capable of sitting down and doing all your work in one sitting; this assumption will only lead to disappointment and frustration when it doesn't come true. Instead, write down a physical timetable that you'll use to structure your work; this timetable will also allocate times when you'll take breaks and have lunch. To avoid clock-watching, consider setting this timetable into an automatic alarm that will alert you when it's time to take a break.
Practice self-discipline. Each little act of self-control you perform will reinforce your work ethic. For example, if it's a nice day outside, you could think to yourself, "Since I'm missing all this nice weather, it means I really must be committed to doing this work."
Use mantras. Mantras have been said to help focus the mind and increase the ability for mental processes, such as concentration. Use these to your advantage; for example, if you start to lose focus, say out loud "Be here now" several times until you regain your concentration.