Most of the time we try to do one of two things when posting yo-ji, hyougen. kotowaza, and other cool stuff:
1. We post something that we learned in conversation with Japanese people. We usually have a story to go along with it, might be able to relate it directly to our lives, we don't have to look up how to use it, and we can rest assured in the fact that it will be useful to us and to our readers because native-speakers use it.
2. We try to post something that lends itself to interesting cultural or historical content: a lot of times we'll look for phrases that are seasonally relevant, or have fascinating origins in Japanese (Chinese) history that will help us and our readers impress people with the range of our knowledge.
But sometimes we run dry, and we have to go yo-ji fishing. When that happens, here's what we use, and now with a little cash and some Amazon.co.jp, you can use them too.
The Yojijukugo "All you need" Encylopedia
published by Escargot Books
This is something I inherited from 大づの先生, the most senior 国語先生 at Kawasoe-Chuugakko. My copy still has his name written down the spine. Sometimes I think he gave it to me so I'd stop asking him what stuff meant all the time. As you can see, it's very, very dense, but all-encompassing, with over 1700 四字熟語 and their definitions contained in its pages. Does not feature example sentences.
Grade School level: Commonly Used Yojijukugo and Proverb Workbook
published by さし書房
I lost the cover to my copy a while ago. This was the first yo-ji book I ever bought, and is still the easiest to use. The Japanese is easy to understand. The definitions are illustrated with pictures that... I guess they could be clearer, but they could also be more obscure. Also doesn't feature example sentences, but it does have quizzes where you have to choose the right saying for the situation or match the definition, etc. Pretty useful.
Increase Your Proverb and Yojijukugo Power!
published by 世界文化社
My yo-ji mentor, Otao-san, uses this book. It's like the Japanese equivalent of a book containing anecdotes or jokes or techniques or quotations to be used in speeches. It contains in-depth explanations of thousands of phrases, examples of the 使い方 for many of them, and is organized into sections based on theme: "Proverbs related to success," "Proverbs for when you're lost," "Yo-ji related to society and nature," and the very broad "Yo-ji related to life" (to name a few). It also has a running sidebar that's like a "On This Day in Japanese History," section. I haven't really looked at that much. Lot of people's names, written in kanji.
Textbook Yojijukugo 2009 Wall Calendar
published by 旺文社
Last but not least, I got this calendar as a house-warming present from the very same Otao-san. I find it extremely comforting because it only features three yo-ji a week, and if a Japanese publishing company can't put out more than that, The Daily Yo-ji's not doing so bad after all. It's designed so that elementary school students can also use it (all kanji glossed), which makes it easy to use in the bathroom where, unless you're Brett, you probably don't bring your electronics with you. And yes, I was given explicit instructions to hang it in my bathroom. When I laughingly promised to comply saying, 「毎日読まれるようにね,」 Otao-san corrected me by saying, 「毎日より、毎回！」