The man from next door says it. My mother-in-law says it. The guy in the grocery store says it. The nurse on TV says it. Seems like everyone says Yoisho! (よいしょ!) It's one of those expressions that appear to be a common part of everyday Japanese life but are not usually taught in Japanese language classes. At least not the ones I took.
よいしょ is commonly classified as a kakegoe (掛け声) word, a call or shout of encouragement to oneself or to others. In Kenkyusha's "New Japanese English Dictionary" the term is translated as "Yo-ho!" or "Ho-heave-ho!" In his classic 1998 text "Beyond Polite Japanese," Akihiko Yonekawa is more specific, explaining that よいしょ is used "when lifting something heavy or beginning something." The example he provides is よいしょ！この箱（はこ）をトラックに積（つ）んだら引（ひ）っ越（こ）し完了（かんりょう）だ (Up she goes! Get this box loaded in the truck and I'll be done moving).
As this example suggests, よいしょ is usually said while doing something and, this being the case, it is clearly a feature of spoken rather than written language. When searching for the term in the Balanced Corpus of Contemporary Written Japanese (BCCWJ), a large database compiled by the National Institute for Japanese Language and Linguistics, I was surprised to find that there is not a single hit. Which means that よいしょ occurs a total number of, yes, zero times, in a hundred million words of written Japanese.