The Public Storage location in Old Town Alexandria is very well-run, and worked well for me for four months. Climate controlled, good hours, and the staff is competent.
The U-Store on New York Ave NE is also relatively cheap, I used it for a month. Not climate controlled, but worked for me while I stored all of my belongings for a month.
With either one, I think it’s good practice to have a friend stop by every so often to make sure everything is in order, but that’s just personal preference.