Hi,

Dips in the roof are caused by a number of ailments all related to each
other. First you'll need a ball of string or a chaulk line. String
the bottom of the ridge to check it for sag. Next, string across the
bottom of the rafters from gable to gable at their midpoints to check
for sag.
Post under the ridge to make it as close to straight as possible,
without causing damage to ceilings or walls(taking into consideration
that too much movement will result in cracks in your plaster).
Cut 2x4's to a length 1/2 inch longer than the height from joist to
rafter at mid-rafter span. Use these to wedge each rafter up by
footing on joist and bending it under the rafter. Rafters will
straighten when these temps are posted under the rafter. Do this for
each rafter pair(front & back make one pair) and check across gable to
gable for staightness.
After a rafter is straight, with the temp still in place, you'll need
to install collar ties at 1/3 the height down of rafter to joist
measurement.
As you can see, this is quite involved and more likely a job for an
experienced framer. Not just any woodworker.