1. Weekdays are better. Check the Unused Whitney Permits 2011
page, and the links there to previous years as well. You can see that mid-week dates are quite likely for getting a walk-in permit.
2. Getting an overnight permit is less likely, especially now through the end of August.
However, those unused permit numbers are what is left at the end of the day. If you get there at the 11 a.m. time for the overnights, or at 2 p.m. for the next day single-day permits, your chances are quite a bit better.