1) Take a Break!
Although this may not sound like the best advice for productivity, sometimes stepping away from a project is exactly what you (and your brain) needs. Whether you go for a walk, have something to eat, or just take a moment to relax, i can guarantee that when you return to the song you'll have more energy and will be much more likely to finish that chorus line that you just couldn't hear before, or come up with that bridge melody that you'd been trying to write all morning...
2) Bounce Your Ideas off of a Friend
Speak to one of your friends about your block. Whether they are a fellow musician or not, the chances are that they will bring new life and light to a musical hurdle that you have been stuck on for some time. And even if they can't offer any musical support... Then the brief respite will be beneficial anyway...
3) Start a New Project:
Whilst this isn't always an option... sometimes the best thing is to pause working and simply start a different project. The change of mental scenery might help you think differently about the original song, or you might realise that you've already moved on to bigger and better things...
Whatever you're working on this Spring, I hope that you can find ways to keep yourself focused and inspired... Until next time!
Some great tips here @Allegra - thanks very much.