I lived in Boulder until just this summer, agreed that Craigslist is really the best way to go. Everyone has their preferences in terms of what is the best place in Boulder to live.
South Boulder/Martin Acres is farther from campus but it is near parks, has its own shopping center (regular supermarket, whole foods, & the SOUTHERN SUN. yum!), and the rooms/houses tend to be bigger, cheaper, and easier to get a pet into. Central Boulder is going to be the smallest rooms, most expensive, and hardest to find pet-friendly, but the walk-to-campus convenience is great. If you want to avoid the undergrads, stay off of "the Hill" (due west of campus), which is where the majority of the sororities/fraternities are located. The Chautauqua area is the most upscale--- I've known people that rented out people's "summer" houses during the school year for a pretty reasonable rate. East Boulder is probably the cheapest, but hardest to get to campus by bus or bike. I lived in North Boulder (NoBo) and loved it, best combination of accessibility to parks, price range, pet-friendliness that I wanted, but it can be a little rough around the edges.
Hold out for something in the 450-650 a month range, if you're paying over that it's probably too much. One other thing to keep in mind is that the Boulder renter's code states that no more than 3 unrelated people can live in a house at one time. This rule gets broken a TON, but if an inspector were to find out it could cause issues--although as I said the rule gets flaunted a bit. Something to think about, but not worry too much about if you find a good place with 3 roommates.
Lastly, if it seems too good to be true online.. it probably is. I've noticed a lot of scammers on CL recently in the Boulder rental section. Just be aware. And buy a bike!!! I found it to be by far the easiest ways to get around town. Again, CL is a good place to go for that.
Hope that's helpful. I miss the People's Republic, it's a pretty interesting place.
