I share your pain! Ugh. It's a frustrating situation. I spent six months looking for a new opportunity. There were many presented, but I didn't feel that many of them were the right one, and that's why it took so long.
With many of them, it was a process of hurry-and-wait. Hurry to get a phone interview. Wait for the next step. Some did a phone screen, and then had budget changes. Others had a long approval process for new headcount that had to be decided by an overseas office, and of course this little factoid didn't jibe with the recruiter's statement that the company wanted a new person "right away".
I'll incorporate Starlight's suggestion. For a certain period of time, you really don't want to close any doors. But at some point when it starts to feel absurd, (i.e. YAY! It's interview number six!!!) I'd just stop responding to e-mails and phone calls. There's no benefit in being beaten into submission before you can even start the job.
By this I mean a process where it's obvious you're being strung along. Some companies are looking for people who are desperate, because when a desperate person actually starts the job, he or she won't balk at "surprise" additionally unpaid responsibilities, or "surprise" salary cuts, or similar. It's just not a good place to start for a job where you intend to stay a while.
Don't get stuck on one opportunity's potential. Keep digging on others. But if it's prolonged, do yourself a favor and take a break for a few weeks if it's starting to get you down; then you can come back refreshed.
Best of luck.