Great creative writing HITs (search on the forum, there should be a panda in the recent pages) an easy $2.50 if it's up your alley.
IMO, I've had the best luck most any time of the year outside of the spring. It's so variable based on the work you're willing to do, what your goal is, and how hard you will actually work toward that goal. I've had my fair share of days (as I'm sure most have) of not making money, simply because I lacked the motivation to do what I needed to do, versus what I would prefer to do.
I definitely find the spring time to be driest, but with MTurk, it truly is a waiting game. My best days/weeks/seasons have been the times that I have been able and motivated to focus, and able to work as often as possible. Finding the balance between looking for work, knowing your reputable regular requesters, and knowing when to stop before burning out, all takes a lot of time...and it's very specific to your own style.
I guess what I'm saying, before this turns into a book, is that you'll get the best results when you are on MTurk as often as possible, and if you use your time and resources as wisely as possible. What that means exactly will be different for everyone.