What's the best way to deal with someone who frequently interrupts and talks over you?

Both my husband and my daughter seem to think that anything they have to say is more important than what I'm already in the middle of saying. Hubby, by the way, will not tolerate being interrupted himself, even if I'm trying to give him a vital piece of information that will render his whole conversation moot. For example, he's venting frustration because he found a button missing on his uniform, but I saw it first, and I've already mended it. I try to tell him so, and I can't get a word in. "Let me finish! Let me finish! Let me finish!" (Yes, he says it three times.) This from the same guy who thinks nothing of cutting me off and talking over me. If I point out that he interrupted me, he'll swear up and down, "I thought you were finished," even if I was obviously still talking. How could he think I was finished? Did I sound finished? Do most people abruptly end their conversations in the middle of a syllab...?

My daughter is even more habitual. I can scarcely ever finish a whole sentence when she's around. She does have bipolar disorder. When she's gearing up to a manic, I don't have a chance, and neither does anybody else. She WILL dominate ALL conversation. That's often how I can tell a manic is building up.

By the way, my husband is not my daughter's father. He's her step-father. But her father, my ex-husband--worst of all. Once, after he cut me off several times in rapid succession and I finally got annoyed about him not letting me talk, he told me he was doing it on purpose because I was trying to bring up a subject he didn't want to discuss.

It seems to happen to me a lot. Just makes me wonder if maybe I'm not valuable enough to be listened to.