• Work hard can only happen when you've defined the goal.
• There are 3 ingredients to great work: Natural ability, Practice and Effort. Pretty well takes 2. The best takes 3.
• Cultivate a sensitivity to what you do, you'll notices when the result starts to drop. it's critical in both directions: either you're being lazy, or you're working too hard.
• Understand the shape of real work, then arrange your capability.

Source: http://paulgraham.com/hwh.html