A common complaint among young guitarists who have been playing for a little while (especially if they're self taught) is that they're stuck in the same old strum patterns and don't know how to break out and invent new ones.  These are a couple simple techniques to help break up your patterns and run into things you never thought of playing.

You can learn a lot through mimicry.  Babies learn that way and it works great for grown ups too.  I can hear the lament now... "Won't I just end up sounding like other people?"  No you won't.  Learning new patterns by mimicry gives you a start that you can develop into anything you want.  In my own playing experience, I usually mess it up royally the first few times anyway, and discover some other pattern I like in the process.  Innovation through mistakes is a beautiful thing.

Maybe you're stuck in an indie rock vein.  Certainly not a style known for rhythmic innovation despite all it's other good qualities.  Go find some music that's entirely different, like Afro-Cuban jazz.  Grab your guitar and sit down with that music.  Pick just one chord, even if it doesn't sound right, and try playing a groove that fits what you're listening to.  If you can't find a chord you like, you can always just use muted strings.  Listen closely to the percussion and bass parts to see if you can create a rhythm that grooves with them.  Don't think, just play.  After awhile, you'll find yourself playing strum patterns that you never thought of.  It's totally a body motion thing, not a brain thing.

Next, try taking strum patterns that you know and tweaking the elements of them.  Guitarists often approach a strum pattern like a drummer approaches his kit.  Different parts of the kit for different beats in the measure.  He may use the low kick drum on 1 and 3, the high snare on 2 and 4.  For us on the guitar, we can use the low strings like a kick drum and the high strings like a snare.  Or flip it over and create something totally different.

Muted strokes, or "ghost notes" as they're called, can add a nice percussion element to your strum.  To get them, simply loosen your grip on the chord and pick the muted strings.  Mix them in with regular strummed notes and you'll get some nice textural changes.

Not only can you change the rhythm, you can also change the part of the chord and the texture to create new ideas.

In this last tip we'll get just a little technical.  Let's take a measure full of 8th notes.  Each "\" is an 8th note in this case:

1 & 2 & 3 & 4 &

That's your basic 8th note strum.  For this exercise, go through and leave out one 8th note each time to create a new pattern. R= Rest.

1 R 2 & 3 & 4 &

1 & 2 & R & 4 &

Try removing each 8th note, one at a time.  Those rest spots can be actual rests (silence), tied notes from the previous strum, or a muted stroke.  Each one gives it a little different feel.

After you've been through all 8 possibilities, start leaving out combinations of two 8th notes.  Then 3.  Then however many you want.  Then take some of those combinations and put them together in two measure phrases.  For example, in the first measure you leave the & of beat 3.  In the second measure you leave out the downbeat of beat 2. 

When you start mixing and matching in this way you'll see how many possible combinations you can create.  And that doesn't even get into 16th notes or triplet which make exponentially more options available.

And if you'd like to learn specific types of stock grooves for a variety of styles (and you should), I heartily recommend "The Rhythm Guitar Encyclopedia" by Jody Fisher.  There's hundreds of strum patterns in there in tons of styles.