You don't, you replace them. Depending on what is happening, try adding a bottle or two of lucas power steering fluid treatment. I'm not much for "snake oil", but have had good results with it.
While is it possible to repair/rebuild a rack, BUT the special tools need to disassemble are expensive. You are better off just to buy a rebuilt unit from an auto parts store.
Rob
Most rack and pinion assemblies require specialized tooling and machining to repair. That is why it is most common to replace the assembly with a professionally repaired or rebuilt unit.
What I did on my Camaro and Honda was add brake fluid. Since it is a hydraulic fluid it is a good substitution. I was VERY skeptical because dot 3 brake fluid looks exactly like water. Throw some in...