Originally posted by: TheLonelyPhoenix
Yes, you will need alternate picking if you intend to play anything fast. All downstroke picking is the newbie way to do things, this should be as easy as breathing for you... its easier to learn than you think it is though, it just requires conscious effort for a little while.
Start by going back and work on keeping the rhythm even on one string with one note.
Work your way up to holding down a chord and alternate picking different patterns in it.
Then go off and play your favorite alternate picking solos.
Edit: Okay, maybe not EVERYthing fast... a few Metallica songs (Master of Puppets comes to mind) are exclusively downstroked, and played faster than most people can alternate pick. There are other examples out there too... but alternate picking should at least be a consistent skill in your repertoire. It leads to better form, more natural-sounding notes (particularly if the song was originially done with alternate picking in mind), and a huge jump in your picking speed.