A couple ways. I am assuming this is an unused attic space above the ceiling. Drill and run a bolt or heavy screweye by the peak of the rafter and just below it on the joist with a bolt or screweye. Then get a comealong to pull it up level or buy some #9 wire and turnbuckles. Attach the wire to the bolts/screweyes with the turnbuckle in the middle. Wrap the wire around it self a number of times making the knot in the screweye so it doesn't slip. Either leave some turnbuckles in permanent or do one joist at a time. Either way, then cut some 2by4s and screw them in for additional support maybe a couple feet or more on each side of the roof center.