Hi Jeff,
I found Earl Haag's book to be the best starting point for me as well. I also recommend taking one of the classes sponsored by the Groundhog Lodges - that helped quite a bit too. Those are really just an introduction though and you'll find that the Haag book is really necessary to learn the grammar. By studying Haag first, I think it helped me in the PG class.
I think your question on learning Pennsylvania Dutch "after the fact" is a good one. I'm not aware of anyone that actually rose to proficiency on their own - that doesn't mean they don't exist, just that I'm not aware of them. From what I've been able to find, there are many people interested, and many that have pursued it enough to learn some basic words and phrases, but being able to hold a conversation is another matter.
For myself, it's helped me to involve my two children in it (aged 10 and 12). However, I've had to take the lead in developing a vocabulary and teaching it to the children. It would really help if I could be more "immersed" in the language. Going to the Groundhog Lodge meetings in my area allowed me to really try out my language skills. There is a dialect get-together group not too far from here that I've been meaning to attend.
In addition to testing out my skills with my children, I've been doing quite a bit of reading, in an effort to expand my vocabulary. Unfortunately, there's not much available in the way of interesting stories that use limited vocabulary and sentence structure. I've been slugging it through "Am Schwartze Baer", one of the issues of the Pennsylvania German Society. The author describes the fictional local people coming into a small Lehigh County bar, their local events, and the humorous adventures of the townspeople. The vocabulary is very difficult and the sentence structures about as advanced as one can immagine! But at least it has enough content to keep my interest. I don't think one can learn the dialect by reading poetry. At least it uses the German-style spelling. After gaining in my vocabulary, I can read some of the "phonetic style" books and follow what they're saying.
Like you, I find expanding my vocabulary a real challenge. By reading from different sources, it helps to reinforce it. After I look up the same word about 3 or 4 times, it starts to "stick". The Stine dictionary is the best for that purpose.
I found that with a *lot* of effort, I can speak reasonably well *to* someone fluent in the dialect. I speak slow, and may need to use some indirect words to express myself (when I find myself not knowing a more direct word). However, when I listen to the fluent speaker, I can only pick out bits and pieces, unless they speak very slowly (which I believe they find very unnatural!). With my children, we can converse back and forth about several things (I'll have to describe my language games sometime).
One thing I'll add about the forum - I've tried to use my best Pennsylvania German when writing in dialect, but I try not to spend a lot of time looking up words and proofing my grammar - so it's likely got many mistakes. But I think if we all get too self-conscious about it, and spend lots of times with grammar books and dictionaries when posting, we'll never write anything. Just my thought on it, but I'd encourage you to go ahead and try crafting some paragraphs as soon as possible.
Machs Gut!
der Tim