So I purchased a spring compressor from Harbor Freight and am ready to change to stiffer rear springs. Any special tricks or things to watch for so I don't hurt the car or myself?

If you are going to stiffer rear springs are they also shorter in length than the originals?
IF your new springs are shorter you will only need your new spring compressor to get the old springs off of your shocks...
the new springs can be installed without the need for the compressor due to their shorter length...

By the way you remove the entire shock/spring combo from the car and then use your compressor to get the stock springs off of the shocks.

Which compressor did you get? The threaded kind or the hydraulic kind? If you got the threaded one, wear safety glasses. Use U-bolts to hold the tool safely to the spring. Don't be surprised at how much work or time it takes to keep turning those bolts. Other than that, its easy. Just be safe, theres a lot of potential energy in those compressed springs.

So what springs are you going to . and why?

Just curious, I am finding my Koni's are overwhelmed by the 200 lb coil overs. I need to get more rebound valved in so I guess I have to send them off somewhere.

Just where that is is a mystery.

I have 180s now and a 23mm front sway bar. As you can see from the photo still hiking up the front inner wheel on some turns. Options were to add a rear sway bar (lots of posts on this site saying not to do it) or adding stiffer rear springs. So I am going with 225s to see how they react.

Old trick: take your spare tire and put it over the top of the shock. Have a guy push down on the tire to compress the spring. Stock springs are easy to compress really. Take the nut off the top of the shock.

If you already have 180 springs you may not need to use the spring compressor at all.