Usually I ask to be hospitalized if:
* I have been unable to sleep in 20+ hours and am still not feeling tired (I realize that some college students pull all-nighters, but this is not the case for me, because I understand the importance of sleep for managing my disorder).
* Inexplicable, erratic, unstoppable crying spells that have lasted ~24 hours
* Psychosomatic symptoms such as vomiting, tremors, nausea, sweating spells, dizziness, disorientation, blurred vision, possible psychosis, etc.

I keep a list of "red flags" in my BP management notebook. My closest family and friends are also aware of my warning signs. Before asking to be hospitalized, I'll usually call a family member and also talk to my psychiatrist.

My support network is sufficiently close-knit that someone will notice when I'm in poor shape (even if I don't, e.g. during hypomania), which provides me with a safety net.

The important thing, I suppose, is to create some kind of safety net so that you can push (or be pushed) to get help. BP needs to be strictly managed, and I don't like taking chances.