Formulaic sequences comprise a large body of language and their mastery is supposed to be a key determiner of language proficiency. However, despite their significance in language learning, they are seldom taught in any principled manner in language classrooms and their correct and appropriate use remains a particular challenge to language learners, even at advanced levels. To address this issue, the current study investigated the relationship between knowledge of formulaic sequences and language proficiency as well as the most efficient method of instructing formulaic sequences. Participants of the study were 42 Japanese learners of English as a foreign language at three different levels of language proficiency (low intermediate, intermediate, and high intermediate) at the intensive English program of a university in Japan. Instruments used for data collection consisted of a reading comprehension book containing a large number of formulaic sequences for intervention sessions and an oral-production discourse completion task adopted from Bardovi-Harlig et al. (2015) for testing knowledge of formulaic sequences as pre-test, post-test, and follow-up test. The study followed an experimental design by administering the pre-test, randomly assigning participants to two equal groups of 'focus on form' and 'focus on forms', conducting intervention sessions for four consecutive weeks (following the noticing, retrieving, and generating process) to teach formulaic sequences to both groups but at different levels of explicitness, administering the post-test immediately following intervention, and administering the follow-up test a period of time following the post-test. The findings of the study revealed that knowledge of formulaic sequences is a strong predictor of language proficiency and both 'focus on form' and 'focus on forms' methods of instructing formulaic sequences are equally effective in developing and retaining knowledge of formulaic sequences. The pedagogical implications of the findings suggested incorporating knowledge of formulaic sequences in every language instruction.