I agree with NoWuckingFurries endorsement of the Harrington books. I've only read the first two books, but they are the primary reason why I'm a winning player. I think the books will be a better investment than a *temporary* membership to a training site for the simple reason that Harrington's books can be re-read.
I put my books in the bathroom, and read a lesson each time I visited. If I made a mistake or thought my attention was waning, I'd force myself to reread that lesson.
Harrington on Hold'em Volume I & II are an excellent base and you will re-learn a lot of good stuff. I read both books twice, picking up a lot of new stuff the second time as well.
Now, that being said, the training sites are good because they specifically cover online poker. With the videos they offer you get to *see* what is happening. Also, you can post your hand history and get personal criticism.
I do not subscribe to any pay-to-access poker training site, but I have learned a lot from them.
What I ended up doing is going to all the poker trainings sites that I could and downloaded their free sample videos. (Some are in Flash, but I have ways of saving those .FLV files). I also looked for videos via Google and on the various other online poker forums
CardsChat has some free training videos (http://www.cardschat.com/f53/).
CardsChat members will critique your hands for free (http://www.cardschat.com/f51/).
These are good starting points, and the resources I use. If I got more serious into poker then the next step would be to purchase a membership at a training site.