If you (or others in your band) lead worship while playing an instrument, you MUST be able to play each song well on your instrument or you will become a distraction to those you are leading.
Practice playing the song without singing.
Pianists / Keyboardists
Plan for the intro, the bridge, the accompaniment, with or without the band, etc.
Practice the song with your eyes closed to see if you can lead without looking at your hands.
Decide in advance where in the song you are strumming, picking, playing lead, etc.
Practice all the strumming or picking styles without singing.
Practice everything without looking at the neck of the guitar or at your hands.
Practice until there is no hesitation moving from one chord to the next.
Memorize the chords.
Memorize your plan for leading the song in worship.
Memorize ALL the lyrics.
Put it together withOUT the band
Practice playing and singing the entire song by memory and with your eyes closed. (Closing your eyes will help you see your weaknesses.)
Add the other instruments and vocalists
Once you are confident in the song, add everyone else.
Make any adjustments necessary as you blend or coordinate with the entire ensemble.
Remember, you have an incredible responsibility to lead others into a time of worshiping our God. Your investment in preparation will allow your instrument to be an expression of worship as you lead.