We just went through the same thing. We had to clean out a bunch of stuff so that our house would show well.
We used PODS and they were great. It is fairly expensive, but very convienent.
They bring a storage container to your place (in 4x8, 8x8, 8x16) and you fill it at your liesure. Once it is full, you call them back and they pick it up. They store the container in their secure and heated warehouse and drop it back off at the location of your choice (in most cases your new residence). The containers themselves are pretty cool; they get lifted and moved by a mobile hydraulic vehicle that mounts on the back of the 5 ton truck they use for deliver and pick up.
The cost for the 8x16 is $327 per partial month that it is in storage. The delivery and pick up fee is somewhere around $100.
http://www.pods.com/