The most common doll repair needed is restringing.  Many dolls were strung initially with rubber or rubber bands.  Over the years the reaction between metal hooks in the arms, legs, and heads, and the rubber has caused the rubber to weaken and either break or weaken.  The obvious answer is to not use rubber bands for this.  There are elastic cords that come in a varity of sizes and strengths for any size of doll. Care in stringing the limbs should be taken to route the elastic cord by a route that will provide the greatest overall length so that the streched cord will provide constant tension on the arms, legs, and head against the body.  Some people when restringing a doll will attach the arms or the legs and then twist an arm or leg in opposite directions to tighten the cord.  This is not the desired way since over time the arm or leg used can get turned in the opposite directions causing all the limbs to loosen up.  It can also have the very undesirable to suddenly "windmill" or spin backwards if the cord had been wound too tight. The hooks on the arms and legs and the loop or hook on the head should also be considered.  These should allow for the cord to slid back and forth through them as the doll is restrung.  This will provide the maximum effect for the cord to pull the limbs tight as trys to retract to its original length from the streched length.  Maybe the best way to learn how to restring a doll is to have one restrung at a doll hospital or repair shop and see how thye do the job.