Subtitles always and without exception (well unless somehow only a dubbed version exists or is available).
I feel this way for many reasons, first of all and most importantly, I like watching films the way the filmmaker intended it as much as possible. Second of all, if I'm watching a Chinese film (for example but you can replace it with any country) it's distracting to hear the actors speaking English and I feel like it is very stupid because no matter how good the dub is, obviously a Chinese film set in China with a story told originally in Chinese does not fit with English language voices. Third of all, I don't feel like I'm even getting an actor's real performance if I watch a dub, imagine watching the best performances of your favorite English-language films dubbed by a completely different actor! And I have had to experience that since I moved to the Dominican Republic because people are fond of Spanish dubs for Hollywood films here and I hate them!
Alcove Audio's point about missing some of the visuals or soundtrack is one I've heard a lot, and while I understand that point of view, I still don't share it. I think naturally we are always going to miss something in a movie, that's why we can re-watch them, and a kind of stupid but true example is that we blink, so we always miss some frames! I don't even mind missing some of the movie because it only means that I can re-watch it and discover things I missed. I've learned to read faster as a result of my foreign film watching too, and I think the only one that made me feel like I was significantly missing something was 8 1/2 (a superb film btw), but I saw that in my early days of foreign film watching.
What foreign films are you planning on watching this summer? Because even though I'm strictly a subs only guy, I think that there are some lesser offenders with different types of films (like most martial arts movies or Godzilla movies and things like that, it doesn't matter as much if it is dubbed).