a common variation on the basic alternate strum is to play a quarter note on the first beat
instread of playing the first upstrum pass your picking hand over the strings without touching them
1...&...2...&... 3...&...4..&...
V. ......V...n...V...n...V...&...
Try this with
Blowin' in the Wind - Bob Dylan
http://uk.youtube.com/watch?v=sUz2OulZ-q4
finally you can achieve a very different effect by playing two chords in one measure
for example "freebird" by Lynard Skynard
G...................D................Em.................................
1...&...2...&... 3...&...4..&...1...&...2...&... 3...&...4..&...
V. ......V...n...V...n...V...&...V. ......V...n...V...n...V...&..
........ if I...... leave here...to-morrow
of course lynard skynard had seven band members including three guitarists but if i was asked to play free bird on lone acoustic this is how i would approach the main chords progression