Back when I was a driving instructor (only 2 years!) I used to visit various DMV's 3 or 4 times a week.
Things have improved for many of them but there has always been a huge difference between locations. Find the best one within a reasonable distance and stick with that one (like G-man and TMoney said).
Never go on Mondays, Fridays or the day after a holiday.
Since I moved to a town of around 150,000 I hardly have a chance to sit down before I get my business done. Most of the time, there's only 1 or 2 people in line.
BTW Part of the reason anything having to do with ID can't be done online is the whole immigration debate and how Homeland Security clamped down of this stuff just when the internet would have taken over the DMV.