There is a ministorage chain in Beijing that is quite good, you can find them by calling 010-52062431. I used them last year, their service is not bad at all. Their customer service reps also do a good job of helping you with moving your stuff.
Ministorage Chaoyang Joy City Store:010-52062431
Ministorage Sanlitun Store: 010-64158602 ,0 10-64158612 , 010-64158622