Aim and Fire
1. Adopt a proactive, confident mindset to articulating the first note of a passage. Often times missing the first note comes from a passive, indecisive state of mind. Make a firm decision about when the note will start and attack that point in time.
2. From a purely phsysical perspective, make sure that you "aim" with your chops before "firiing" with your air. One of the main reasons why a lot of first notes are missed is because the embouchure is not set and stabilized before sending the air forward to start the note. Inhale, then set the chops for the desired note, then send the air past the chops. When done correctly, there will be just the absolute slightest hesitation between inhale and exhale to allow time for the embouchure to stabilize.