Re: Etiquette for visiting foreign dojo
I've done this quite a few times now. It's courteous to email or call in advance, but in cases where I couldn't do that (no email address available, etc) I've also been welcomed just arriving early and politely asking to train.
It's a bit easier if the other dojo is the same general style/association as yours. If you visit a dojo of another association, be prepared to feel fairly clueless. Cross-style weapons practice is particularly hard as everyone seems to do different weapon forms.
It's a good idea not to engage in comparisons between the dojo, even if your hosts seem inclined to do so. Just take the new experience for what it is and try to learn as much as you can. Also, especially if the styles are different, you'll be told some things that flatly contradict what you learned at your own dojo. Bite your tongue....if you're somewhere else, you're there to learn what they have to teach. (I screw up on this occasionally, not being tempermentally inclined to bite my tongue, but I always regret it.) If you've just got to say something, a diplomatic solution is "Please be patient with me on this one, we do it quite differently.'
It is a good idea to have your own sensei's permission to practice elsewhere. I've never gotten a letter of introduction and frankly would be embarrassed to do so, but I do let my teachers know and give them the opportunity to say "Heavens, you'll get hopelessly confused" if they want to. (So far they haven't.)