Dual citizenship
I too have dual (UK/Swiss) citizenship. I tend to use my UK passport for the Western hemisphere + Japan, the Swiss for Eastern hemisphere and try and be consistent, so I don't end up confusing immigration officers. On top of that getting visas into my British passport can be tricky, when I'm resident in Switzerland and can't supply a resident's permit (which I don't need, as I'm a Swiss citizen.
One curiosity is the place of birth, which most countries have. Switzerland doesn't. It has a "place of origin", which basically is the place where your family comes from and generally doesn't change within a family. Can end up confusing immigration officers, but most of them don't spot the distinction. I've never dared entering the US with my Swiss passport, with my I94 giving a place of birth in the UK and my passport saying something totally different (though I guess relatively few Swiss are actually born in the place of origin).