Herm...if Bruce Lee said it, then you're probably better off getting it in Chinese rather than Japanese...in addition to being more authentic, it will be more compact and thus better suited for a tattoo. That being said, here's my shot at the t/l--"let go" and "when you least expect it" have a lot of subtleties in English here, so someone please correct me if I missed them in the Japanese.
逃げないで、放ろう。求まないで、知らず知らずの内に来るからよ。
Phonetically, it reads 'nigenai de, hourou. motomanai de, shirazu shirazu no uchi ni kuru kara yo'.