I've been at it for 3 years now. I cannot tell you what works.
The best spike in sales on existing books I have ever gotten, was with the release of a new book, so I maintain that addage. The best promotion is a new book.
Blog tours, book forums, etc... all seem to be authors advertising to authors. Very few readers participate in these yahoo groups and blogs. Review sites gain a trickle of sales, but if you have to buy them a book, its often not a gain in the long run for the amount of sales it will garner. I've bought advertisements and they yield a trickle of sales, often not mounting up to the cost of the ad.
The very best trick I have learned is putting a book out free, then when its amazon rank climbs, then switching it to a price and the sales roll in for awhile. As your rank dwindles though, they come slower and slower. So, for Amazon, rank is everything IMHO, if you do not have an established readership. Which for a bottom tier author like me, I have only a small readership upon which I can rely.
All my efforts through what seem to be the normal channels of my peers has been fairly fruitless. Time is a huge issue for me, as I have hardly any and spend what litttle i have writing, rather than promoting. So when I do promote, I need something that produces.
Last thought (I tend to ramble) ... after ceasing all efforts to promote my books, my sales continue at pretty much the same pace. New releases bring a nice spike for a month or three and then levels drop back to normal.
I am currently working on a new tactic. Anthologies. By getting my name in anthologies with higher tiered authors, I hope to gain some exposure. My first will come out in August, so I'll be watching for sales spikes on my backlist and let you know the results.