I start giggling out of nowhere too a lot during the onset of mania....sometimes I'll start laughing at really inappropriate times too. This always scared my mother.

Another one others here mentioned is hypersexuality.....this a definite "warning sign" for me too, especially when it leads to promiscuity. I've made some really bad decisions I look back on. Starting to spend more money too on anything: items, people, etc. is another warning sign.