First I'd like to put out that Shake is a great compositing program (I've been using it for 4 years now), but its a dying program. Its a good program to learn node-based compositing but it's been fazed out by Post Production houses as support and updates are a thing of the past. Hopefully Phenomenon or whatever reincarnation is supposedly going to be released in the future goes above and beyond Shake and all the current competition.
Although I'll agree with you that Shake has been EOL'ed, I'll have to respectfully disagree that it is being "phased out of post houses".
Shake experience, at varying levels, is still sought after at many shops of varying size because it's such an integral part of the pipeline. Now that the SDK is available, larger shops are able to broadly customize it to fit their needs.
Are there other, more technologically advanced compositing apps available? Yes, absolutely. Nuke is among them. But Shake still performs well and gets the job done. There may be a day when Shake is no longer used, but with some freelancers still using Shake 2.5.11 on Windows, you can see it's lasting effect.
To the Original Poster, I suggest purchasing a copy of this book:
http://www.amazon.com/Apple-Pro-Tra...bs_sr_1?ie=UTF8&s=books&qid=1204564342&sr=8-1
It will teach you the basics, and some advanced topics, but realize that compositing is not all about the software. There's a technique to it and an eye to it. Recognize how things look in the real world and how things react in the real world. Study photography. Re-educate yourself on color theory.
Apply those skills to your technical compositing knowledge and in time, results will come.