Do NOT let all the fluid drain out when preventing such is so easy. (Not to mention clutch is still fed by master cylinder...)
Either cap off line at each corner w/ a golf tee or wire nut. Or if replacing brake lines in spring then just crimp each with a small a clamp or pair of locking pliers. Some will still drip using either method so wrap w/ a zip-lock bag if worried about any spillage.
Good luck
