I just moved from Florida to California and may be able to answer some of your questions.
I actually went to California and stayed in a hotel (with as many personal belongings as I could take with me on the plabe) while I was apartment searching. I did this because of the housing market in Berkeley. I was told by many locals and realtors that, generally, housing in Berkeley becomes available immediately, so it is best to be ready to move in as soon as you locate a place you like. I used www.craigslist.com
to look for potential apartments while I was still in Florida. I saved those listings that looked interesting and set up appointments to see them upon my arrival in Califonia. It's best to look at a variety of places so that you can get a feel for neighborhoods, rental options and prices. Keep in mind that a lot of times (this is especially applicable in Berkeley) that higher price doesn't necessarily equal better apartment. So take your time and shop around. For those moving across the country, I recommend furnished apartments, if those are available in the area. (Let's face it, you probably won't want to take the cheap furnishings you aquire in law school with you when you buy your first home as an attorney. So why waste the money for things you will only use for three years?) I was lucky enough to find a fully furnished apartment for a very reasonable price just 4 blocks away from campus.
I spoke with my landlord and told them that it is very likely that I will be taking a summer clerkship somewhere other than Berkeley, and they were very willing to accomodate me. They changed the lease agreement to allow me to sublease during the summer as long as they approve the potential tenant. As far as getting a place in the city where you will have your clerkship, I would look for summer subleases. There are plenty of college kids that leave to go home for the summer and families that take vacations at their summer homes. I don't think you will have much trouble finding a place to stay. Plus, the firm where you will be working will probably suggest some housing arrangements. (You will be making a nice chunk of money, so cost isn't likely to be a problem.)
Hope this helps.