Back in 2008 I was writing more than 50 articles a day. That means I was getting up very early in the morning and writing all day long. Now I never write more than 30 articles a day but I am thinking about raising the number once more to earn more money.
To stay sharp and focused is simple. First start with some kind of meditation, think of all the work you have ahead of you. Mentally rehearse all the articles you are going to write and think about how many people you are going to help with them. When people write with a mission they have more mental power to carry it on.
Second, buy some energy drinks and drink one. It is very important that you do not drink more than one per day or the brain will suffer. Just one energy drink is enough to make you more alerts and to write faster than without one. Redbull is very expensive so I recommend you to drink some cheap competition energy drink that has exactly the same ingredients, caffeine and taurine but with the difference of not having the same pack and color and a bit different taste.
Write using a good text processor because when writers use out of date text processors they will not have the same motivation to write. The best text processor is Word 2010 but if you need keyword count and density you should use Article Jockey.
Finally try to write an article under ten minutes. A small 400 word long article can be easily written spending less than 15 minutes. Between ten and fifteen minutes is the ideal time to write a top notch article.