In my experience, more advanced learners of Japanese often lament the fact that they didn’t start to learn some tougher stuff right out of the gate. For many, their white whale is kanji, but for me, it’s keigo. My choice could have been influenced by the fact that I used to work in an agricultural high school, where keigo was about as useful as a lead balloon, but I nevertheless regret it. Especially since I’m ok talking about everyday topics with the (wo)man on the street, but I sometimes can’t understand what the clerk at UniQlo is asking of me.
For those of you, like me, who are trying to play catch up, here are a few resources that may assist you in learning the humble (謙譲語 – kenjougo), -masu form (丁寧語 – teineigo), and respectful form (尊敬語 – sonkeigo). It’s tough going for sure (here is an article about navigating the keigo “minefield” from the Japan Times), but well worth it.
My first resource is a nice little explanation at The Japanese Page. It explains when to use each of the three styles of polite language, and provides a very basic list of common verbs in each style (complete with pronunciation help). Another good site is a Guide to Japanese page, which provides a nice explanation if its own, as well as a more detailed explanation of the intricacies of polite Japanese speech.
If your Japanese ability is a relatively high, but you’ve never gotten around to learning keigo, there is a nice series of explanations in Japanese at the Agency for Cultural Affairs.
Finally, if you’re willing to shell out some money, JapaneseShock has a keigo cheat sheet for sale. I haven’t shelled out the money for it, so if anyone has bought this tool, please let me know what it’s like.
Hopefully these resources help. Good luck!