Sumimasen (すみません) is possibly the most versatile phrase in the world (or at least in Japan).
It's translated in my guidebook as "excuse me," but its use in Japan seems much broader, kind of like a combination of excuse me/I'm sorry/please/over here. I've heard it more than any other phrase in Japan, in situations ranging from matcha tea refills to catching someone's attention to handing over an object to apologizing for not understanding English.
On the other hand, another phrase I expected to be handy was "ikura desu ka" (how much is it), but the first time I tried asking it, at a small store, I immediately realized how useless it is to me. (The Japanese shopkeeper promptly announced the price in Japanese numbers, which I, of course, did not understand.
Oh well, as the Japanese say, 七転八起, right? Wait, that requires numbers, too...