There are lots of people who use and develop with InfoPath and many of them post tips, tricks, etc. to their blogs.
One that stands out, in my mind, is actually an entire company that is devoted 100% to InfoPath consulting and solutions. InfoPath is ALL they do.
The company is Qdabra Software and they share their expert InfoPath knowledge on a public site they host named InfoPathDev.com. On this site, there are well over 100 "How-to" articles, tips and tricks. And, they don't write articles about "101" InfoPath topics. They write about the difficult issues that we all face when we try to implement an InfoPath solution in the real world.
There is a reason that Qdabra has all of this deep InfoPath knowledge. The company was founded and is owned by Patrick Halstead, one of the original development managers at Microsoft on the development of InfoPath v1. Also, Patrick is the author of the best advanced-level book on InfoPath 2003, Developing Solutions with Microsoft InfoPath. (If you follow this link to Amazon, you will find that the book has a rating of three-stars out of five. Don't be fooled by that. I've read the entire book from cover to cover and have used it extensively to develop InfoPath-based applications. It is not perfect, but it is by far the best text out there on advanced usage of InfoPath 2003).
So, while there still is a real shortage of expert-led classroom-style InfoPath training, there are some very good resources available on the Internet and InfoPathDev.com is one of the best.
(If you go to their site, you will see that a lot of the most recent tips are written in Japanese. If that is not your preferred language, keep searching their site because the tips in English still far outnumber the Japanese ones. The reason they are posting a lot of new tips in both Japanese and English is because InfoPath is BOOMING in Japan. :))