trublue - thanks for the mention.
I'll add a few thoughts.
1) Permanent Frees are harder to get into Top 100.
2) If you can get them into Top 100, they can start there for a long time.
3) It's much better, at least the first 1 or 2 times, to stack together promotions. If your book is recently permafree'ed then most of the sites wouldn't have found it yet.
So stack up sites and try to get your book into the Top 100.
Stack up means - same day.
Later it'll become much tougher because readers would have found it.
4) The small sites that mention for free are a big help. Apply to as many of them as you can.
5) Later on, after a few months, it's good to have 1 or 2 big sites a month. If you have a good cover and a good book, then the tail will be long, it'll tide you over until the next month.
6) There are lots of promo results threads. Find one for your genre. For the first time, don't space out promotions. You need somewhere between 2,000 and 3,000 to get into Top 100 (because Permafree it's tougher). So stack enough sites on Day 1 of your promo run that you can get that. If you get to Top 100 throw in a few more sites.
The ones that work best (pretty similar list to what trublue said) i.e. BB, Freebooksy, ENT, Bookssends, book butterfly, and book gorilla. Note: I don't know about Book Gorilla. The rest are very solid.
BB, Freebooksy, ENT - those are I think the top 3 for freebooks.
Booksends, BKnights are also good. For us let me know genre (via pm if you prefer) and I'll let you know if we're strong in that.
Also MidList accepts free promotions.
BB you might have to apply a few times. Try a few times (you don't have to wait 4 weeks for free books). If you can get in there, then stack everything around them (the day before).
If not, get Freebooksy & ENT on same day and stack everything around them (same day).
BKnights is pretty much automatic since it's $5 if you mention KBoards. Other sites you can see what budget you have left. The small free book sites are very important. Sometimes they can get you as many downloads as a big site.