1. Change the Tempo
This is such a simple thing, but it can make a really big difference in the mood and feel of a song. Maybe that dancefloor stormer you're working on would really work better as a ballad. Or would that slow, contemplative track benefit from the increased energy levels a higher tempo can provide? Doubt this would make a difference? Keep in mind that both New Order's "Blue Monday" and Depeche Mode's "Enjoy the Silence" were originally intended to be ballads. After speeding them up, both turned out to be not only some of the biggest hits either of those bands had, but indeed some of the biggest hits in popular music period. A tempo change can make for an interesting remix idea, too. Front 242 did an instrumental remix of their track "Religion" on the maxi-single of the same name that was simply the track slowed way down. You'd be surprised how different the song feels with just a simple tempo change.
2. Frankenstein's Monster
I'm a musical packrat. As I've mentioned before, I always have an active 'ideas' folder on my computer where I can quickly store musical ideas that I can later come back to and hopefully flesh out into songs. Of course, a lot of these ideas end up going unused either because on second listen, they aren't that inspiring or maybe they just don't fit the sound of a certain project. But no matter how sure I am that an idea will never make the cut with me, I always hang on to these little song snippets. There are a lot of benefits to doing this, but one solution they can provide is a method of getting up and running again when you're stuck. Sometimes the verse structure of a song comes out great, but you can't figure out an appropriate chorus to go with it, or vice-versa. Having a library of unused ideas allows you to audition some of them in just this sort of scenario. Sometimes an idea that seems "blah" on its own takes on a whole new life if inserted into a different context.
3. Shuffle Your Sounds
Many songwriters underestimate the importance of arranging when it comes to how a song sounds. The sounds you use and where you place them make an enormous difference in how a track sounds. If you're stuck on a song, try saving a second copy of the song and going through and changing every single synth and drum sound on the track. You can always go back to the original if this proves fruitless. If you have doubts about this, try tracking down some old Depeche Mode demos or the demos from Nine Inch Nail's "Pretty Hate Machine". Some of them sound painfully lame before the right sounds and studio magic were applied. Don't be afraid to get crazy and experiment with sounds you wouldn't normally use, either. Sometimes the 'wrong' sound in the right context can turn out amazing.
4. Get Lost
Although I am primarily a keyboard player, I also have a bass and an electric guitar. I'm not a very good bass player and I am even more useless with a guitar, but I like having around a couple of instruments I am not as familiar with as the keys. When you've been playing an instrument for a long time, I think it can be quite easy to get into a rut and have certain 'default' intervals, keys, or chord shapes that you fall into time and time again. Having an instrument on hand that you are not as familiar with almost forces you to go in different directions than you would on your 'home instrument'. Try some unusual stuff too, like mapping your drum pads to trigger notes on your favorite synth. Anything to break you out of your normal way of playing can be a really valuable method for coming up with new ideas.
5. Move On
When you're stuck, probably one of the worst things you can do is to just sit there listening to your track over and over again hoping some miracle happens and a great idea pops into your hear. You're only going to get sick of the track and feel less inspired to finish it. So if you've tried a couple of the above ideas and it still isn't happening for you, take a break and work on something else. Sometimes you just need a bit of a musical 'palate cleanser' to reset things. When I'm working on an album, I usually have at least 4 or 5 songs I am working on at the same time. That way, if I get stuck on one, I can simply move to another and still be moving forward on the album. I realize this sounds pretty unfocused, but it's worked wonders for me and I often find that after working on another track, I'm more eager to come back and work on the track I was stuck on in the first place - sort of a creative 'reset' button. Remember what they say about absence making the heart grow fonder.
So what about you? How do you get yourself unstuck when you're at a creative standstill?