Why are word sprints important for rough drafts? They give you relatively quick gratification. By the end of the day, you not only feel accomplished, but you drastically improved your position in the plot. It makes for a great motivator, because, for example, if you are only at 20,000 words on Monday, after a word sprint, you may be at 25-30,000 words. It pushes you days ahead of your routine, which helps alleviate some of the pressure.
The best strategy for a word sprint, at least for me, is to spend the night before lightly mapping what you will be writing. The following day, give yourself an absolute minimum amount of words you wish to write (for me it's 6,000). You want to hold yourself accountable to the sprint and setting a clear minimum will help push you if you find yourself starting to fade.
I have written a couple posts about the importance of finding your writing routine, and adjusting it to your life. Currently, I am in the process of adding one word sprint a week to my routine. Based on my findings, I have discovered one sprint per week is the ideal number. A sprint can be rather draining. By doing it just once, you snap back into your routine the next day, and most likely will not skip a beat. Adding a second sprint can be beneficial, but I've discovered you're more likely to skip one, or two, other days that week. Again, these findings are by no mean scientific. I am not working with a test group. It is purely trial and error on my part.
If you currently are looking for a way to spice up your writing routine, try adding a weekly word sprint. At the very least you will realize you don't like it after adding 5-10,000 words to your manuscript.