Instruction
Minnesota

Everyone can benefit from lessons from a good teacher, preferably one-on-one.  I've included lots of resources for the "home study" approach, including video lessons, at the FolkBlues main instruction page.  However, you should be able to find some live teachers wherever you live.  Try local music stores, or talk to the band teacher or music teacher at local schools.  If you live around the Twin Cities, there are countless local music stores which will help you arrange lessons.  Here are a few good places to hook up with a good acoustic teacher.

Homestead Pickin' Parlor
Without a doubt, when it comes to straight acoustic folk or bluegrass, Homestead has the broadest range of teachers, drawn from the ranks of the finest local performing musicians.   And if you can't find what you want on their roster, they can probably hook you up with someone who meets your needs.
Minnesota Bluegrass and Old-Time Music Association
MBOTMA publishes a newsletter, which lists teachers of fiddle, banjo, guitar and other instruments.
Podium
The Podium is the place if classical guitar is your bag, and it also has several fine jazz and folk or blues players ready to offer lessons.
West Bank School of Music
The West Bank School of Music has been around for lotsa years, and offers private lessons, group classes, and student ensembles, taught by professional musicians.   A variety of instruments, including some brass and piano, mixed in with traditional acoustic stuff.
MacPhail Center for the Arts
A pretty highbrow operation, "MacPhail Center for the Arts strives to be the best comprehensive community resource for education and performance experiences in the musical arts for people of all ages, abilities and economic circumstances."  Another good resource for those wanting to develop serious musicianship.