Picking is a surprisingly difficult skill to master. To become adept at it you will need to be able to play up and downstrokes in a variety of combinations. The main picking combinations that you will need to master as a guitarist are alternate picking and downpicking.
Alternate picking is where a downstroke is always followed by an upstroke and vice versa. It is an essential skill for fast lead playing.
Downpicking is the use of downstrokes only. It is hard to play fast riffs using downpicking but it does produce a more consistent sound and is used alot in punk and heavy metal music.
The table below shows all the combinations of up and downstrokes for a four-note sequence: (U = upstroke, D = downstroke)
|D D D D||D U D U||D U D D||U U D D|
|D D D U||U D U D||D D U D||U U U D|
|U D D D||D U U D||U D U U||D U U U|
|U U U U||U D D U||U U D U||D D U U|
Play through figure 1 (below) using downpicking and then the two forms of alternate picking (down up down up & up down up down).
Once you've mastered downpicknig and alternate picking you may want to experiment with some other picking combinations in the table above. These other combinations are of little practical value however they do stretch your skills. One other combination you may have encountered before is up-picking (using only upstrokes).