Lowering Rear End

How do I lower the rear end on a 1985 911?

Your 911 has a torsion bar rear suspension. To lower it, you will need to remove the spring plates, and reinstall them on the torsion bars in a different position, with the trailing(back) end up closer to the car. The spring plates are the flat metal pieces that go from the torsion bars, back to the axle. If none of this makes any sense, check out the Haynes repair manual for 911. You may want to have a professional do the work, but if you do it yourself, make sure you mark the position of the spring plates before and after, to make sure they are the same left to right. Also, make sure the car gets a four wheel alignment after. I myself have never done this, but it doesn't look terribly difficult for a competent mechanic with the right tools.