advice on choosing a martial arts school
First off, my specific recommended list in the San Francisco Bay Area is:
However, it's probably the case that you live and work somewhere where this is not practical for you. Following is some advice on choosing a martial arts dojo, and specifically an aikido dojo. (Obviously this is all from my own perspective, but if you didn't want my opinion then you wouldn't have asked, or you wouldn't be reading this.)
General Martial Arts Advice
If you have never trained in a martial art before, and you are not experienced in watching martial arts training, it may be difficult to correctly interpret what you see when you visit. This is one reason why you should try to get a friend to take you to his or her dojo: you'll have someone with whom you're already comfortable to ask questions about what's going on. Barring that, you should look for a place where you feel comfortable. For me, this includes a feeling of being welcomed. If people look like they're having fun, I usually also take that as a good sign. (Training can be serious, rigorous, and fun all at the same time.) Talk to some of the students and find out what they like and don't like about their school. See if they look "professional" when they practice. Talk to the instructors too, to see if your goals are in concert.
If you hear things like "just sign this contract promising to pay us for <however many> months" or "guaranteed black belt in <however many> years," say "thank you very much" and leave. Those are signs of places whose primary interest is in staying in business, or growing their business, and not in training proficient martial artists. You will most likely be asked to sign a waiver wherein you say you realize that martial arts can be dangerous and that you promise not to sue the dojo if you get hurt. In my opinion, that's okay. Be respectful of your body and your limits and you will probably not get seriously hurt. You can plan on the occasional bump or scrape, though. As the teacher of one of my friends is often known to say when this happens, "we not doing flower arranging."
Another thing to look out for is a martial arts school that professes to teach a long list of arts. You'll be better off learning from someone with serious focus. If you want to study aikido, my advice is to pick an actual aikido dojo, not "Throckmorton's Martial Arts Studio and Type Foundry" where aikido is one of thirty subjects all taught by High Grandmaster Throckmorton. Also beware of people who call themselves founders of their own martial arts. Although many respectable arts do have individual founders, in the USA right now you are more likely to run into some sleazebag who invented his style based on inadequate training and stuff he saw on TV, as a marketing ploy to get more students or to inflate his own ego. In the general case, my advice is to avoid these people and their schools.
If you've narrowed it down to a few potentially-acceptable dojos, to be thorough you may wish to sample each of the instructors teaching at each dojo, if possible, since you may find that you have radical differences of opinion about teaching styles of instructors within the same dojo.
My aikido dojos have always been part of the organization called the Zaidan Hojin Aikikai, which is the organization considered to be "main line aikido". It was founded by Morihei Ueshiba O'Sensei, the founder of the art itself. According to my understanding, it represents, as best an organization can, his conception of the art at the time of his death. Ueshiba's grandson Moriteru Ueshiba now is the doshu ("leader of the way") of the Aikikai. It has evolved, of course, but part of the goal is to evolve it in ways in concert with O'Sensei's final philosophies.
There are lots of politics involved with aikido organizations, but there are also some fundamental differences in approach. Sometimes the differences are based on the current conceptions of aikido as extant at the time of the organization's break with O'Sensei, and sometimes not. My personal preferences, of the styles about which I know, lean heavily toward the one I am practicing, unsurprisingly enough!
Of the few other aikido organizations about which I have beliefs, there are "Ki Aikido" and "Yoshinkan Aikido". From what I understand, the Ki Aikido practice focuses on extension of ki, apparently often to the exclusion of martial technique. Yoshinkan Aikido, again according to my imperfect understanding, focuses on teaching the precise angles and steps (e.g. "put your left foot exactly here or you're wrong") which compose each technique, from their view. (That sounds pretty silly to me, since the exact angles depend on body sizes and flexibility, and the exact steps depend not only on that but on motion and speed as well.) Both of these approaches seem less satisfying to me than my own, but I have never been to classes in these styles, so I could be pretty far off-base. There are other aikido organizations, and some dojos aren't affiliated with any of them, so it can get pretty confusing. So my real advice is not to worry about affiliation, but instead concentrate on finding quality.
By the way, it is usually the case that the rank of your instructor will influence your experience learning the art. Yes, sometimes organizational politics can influence the rank which someone holds, but as a general rule it's a fairly useful metric. It is also something that grows in influence as your training progresses. (In other words, having a lower-ranked teacher is probably more acceptable for beginners than for advanced students. This is not meant to imply that basics are unimportant or simple, though!) Of course, everything ultimately depends on the individual situation.
Good luck! Please let me know if this advice proves to be useful (or not) with your own experience.
Nick Walker has also written an excellent essay on this same subject. Nick's essay is much better than mine, but I didn't know about it when I wrote this.