Keep in mind:
1. http://leadandcoal.com if you're renting blind, you may want to consider how close you'll be to construction (you may be able to negotiate a good deal-- lots of available rentals around the construction zone) They're finishing up Coal probably by early August, then starting on Lead
2. I see a lot more rental signs than I've seen postings on Craigslist, so there is some benefit to being here in person.
if you can afford to make a scouting trip or to rent a month-to-month when you first arrive, you might be better off. in my experience, some of the older houses in the university heights and silver hill neighborhoods should really be seen before you decide to rent. (nob hill tends to have somewhat "nicer" rentals, but the rents are correspondingly higher.) there isn't quite a season but there are lots of rental places to be found by simply cruising around the neighborhoods you're interested in and looking for signs out front. also, don't forget to check out the neighborhoods north of lomas near the law school and golf course. if you can swing the rent, there are some nice places.
for month-to-month I think someone on a previous rental thread recommended the girard apartments, which are quite close to unm, north of campus. this is the only link I could find. good luck!
For what it's worth... I'm renting in Nob Hill. My landlord, who rents usually to college students offers a 6 mon. lease and requires that the lease run from June to January or January to June only. He says that's when the college students are looking to rent. The house I"m in has it's quirks, but its 4 bdrm, 2 bath and adequate for our needs.