Haven't posted at HomeRec for a while (of course, I've been lurking), but this thread inspired me to chime in. It's a subject that I think about a lot, since I'm always trying to do new things with every song I write. I've only been writing music regularly for about 5 years now, but I've built up a list of things that I do when I get stuck on "sameness" in my music. They're not necessarily things you'd want to do all the time (otherwise, they wouldn't be new), just things I try every now and then when I get stuck. Maybe they'll help you, too!
In no particular order, just as they come to my head...
1.) Stop thinking about chords in terms of chord names and theory. Gmaj/Em/Dmaj? Forget it! Pick up your guitar (or sit down at your piano, or whatever) and start strumming weird stuff, anything at all, with fingerings and positions you never thought you'd ever use. A lot of it will sound like crap, but every once and a while you'll stumble on something brilliant...and that might be all you need to help you make something new. You'd be surprised at how good some chords sound that you'd never ever hear of in a theory book...
2.) Do you usually think of a melody first, then build the chord structure? Do the opposite. Same goes for vice versa. Breaking up the routine breeds innovation.
3.) Work in arpeggios. This'll help emphasize the individual chord members as opposed to the entire chords, and encourage you to do things you normally wouldn't do, such as dissonances or other combos.
4.) Work in a different time signature. Maybe even 3/4 as opposed to 4/4. Or, do something really wacky...try writing a melody in 11/8. Mix and match normal time signatures for strange things...i.e., doing a melody that alternates 4/4 and 5/4, or something like that.
5.) Write in a normal time signature, but using weird rhythmic patterns. For example, try writing a melody that relies entirely on upbeats.
6.) Like the way a specific rhythm pattern or chord change sounds in a song you like? Steal it. More specifically, take it and put it in an entirely different context, and see if you can build something new and unique. The simpler the element, the easier it is to make it into something new. Listen to lots of different types of music...
7.) Write with different instruments. If you usually start off with the guitar, write the bass line first. Or the keyboard riff. Or even the drum part. Or use an instrument you normally never use. Anything goes.
8.) Mix up the roles of your instruments. For example, have
the bass drive the main riff or melody. It'll make it sound very different.
9.) Mix up the length of your voicings a lot...you don't always have to have 4 chords that each last 2 bars. Use passing chords to transition from longer chord blocks, have some chords last longer than others, etc. It'll break up the predictability of your progressions.
10.) Work with different phrasings. For example, you might write a melody in 4/4 that lasts for only 3 bars, or lasts for 5 bars. Or whatever you want.
11.) Get off the "verse-prechorus-chorus-bridge-chorus" thing. You might try writing a song that has absolutely no repetitive melodies, or a song that's based entirely on a repetitive rhythm or melody, and see if you can make it interesting...
12.) Write several different melodies and have them all go simultaneously. This is hard to do correctly, but sometimes it works against all expectations.
13.) If you're a guitarist, use different tunings, of course. But also, writing in bar chords or power chords sometimes helps a lot. It sounds really cool, and you'll sometimes use chords changes that you'd never do otherwise. See if you can build fuller chords off of a power chord progression.
14.) Try writing a song when you're really drunk. Or high. Or caffeinated. Just don't get arrested.

15.) Think of a specific mood or atmosphere that you want to convey before writing anything. Then, try writing a melody or chord progression that fits that mood. Or write music to fit lyrics if you normally write in the other order.
16.) Try imitating a specific genre or style. Be like, "I wanna write a song that sounds like '30s big band," or "I wanna write a string quartet." Sometime it won't sound like the target, but it might still sound cool.
That's all I can think of right now...it's a long one but I hope it's at least a little helpful to anyone who might be looking for some wacky new ideas.

Good luck!
-Derrick