Used to have this problem too, and come to think about it, I still do have this problem. I swear it's the product of an overactive imagination, but that can be good or bad, depending on how you utilise it.
You could follow Chance's advice about incorporating the elements of your new idea into the current fic for a suitable twist. However, if you're like me, you would be thinking of entirely different settings, scenarios and characters, changing permutations here and there. I've done the same: writing ten, eleven chapters then giving up rather suddenly and leaving the fic to hang in a limbo. I've done this for fifty or more.
It is a natural process of writing to abandon your pieces of fiction halfway. It does contribute to making you a better writer as compared to slogging and forcing yourself to finish one piece of fiction. Somehow I feel that it will eventually lead to a smoother flow of ideas in relation to the main theme of your current piece, and that you don't get as influenced by other ideas as much anymore. However, in this time of frustration, I've learnt to immediately write (rather detailed) synopses of the idea I'm having into a word doc and saving it under a new fic. This helps to flesh the idea out and build a minor skeleton around that rad idea you just thought of, getting it out of your head at the same time.
Then, on days where your inspiration for your current fic runs dry and you're waiting for it to return, you COULD revisit the synopses and work a little on them, but not to the extent of writing a new piece unless you're more or less confident that you can handle more than one piece at the same time. Try it out. It may or may not work for you. Perhaps Chance's idea is more suitable in your case, but you never know until you try
Also: this thread is almost 2! Heh.
