How do you learn to live with someone with bipolar illness?

One good way is to ensure he (or she) is taking his meds. If he really is bi-polar (formerly known as manic-depressive), he should be under a doctor's care and taking the meds needed to control his illness. If he is already on medication, you must do everything in your power to encourage him to stay on them, even when he's "feeling great and doesn't need them." To do so is not nagging. You're saving your relationship -- and maybe HIS LIFE!