One WIP at a time. There's a reason I have that rule, a very important reason. When you're writing your novel and things get hard, your imagination is your worst enemy. When people talk about ideas of how to get inspired, they're ignoring a greater problem: only writing when you're inspired. You need to be able to write even if you're not inspired. There are going to be entire chapters that don't thrill you. You just have to slog through. If you only write when you're inspired, you'll finish a tiny fraction of your proposed manuscripts.
Things were kind of tight on Thursday and Friday for my WIP. So what happened over the weekend? I had ideas for THE RED SOCK SOCIETY, THE SEVENTH SACRIFICE, and WHAT'S BEHIND THE CROOKED DOOR? I was incredibly creative on every project except the one that needs my attention.
Why? Because inspiration is easy. If I write to inspiration, I'll die with a thousand half-finished novels. No, I need to pocket those other mss for when their time comes and buckle down on the one that has 50,000 words and needs another 50k so it can be complete. I want to finish this thing so I can start work on one of those others and finish that one and so on.
It is the substance of a writer that he can write. Period. Not write when the mood strikes him or when it's easy. You understand your craft and you can take it to the appropriate conclusion. New ideas are like will-o-wisps. It's easy to get lured off by the exciting and easy, but when you've sated your writing enthusiasm on this new wellspring, you find yourself standing in the middle of a swamp, sinking in the mire of incomplete work.
Focus. Prove your mettle. Finish what you started. There will be time for the next work when you're done with this one.