Spent casings or clothespins do the trick just fine. I've also seen people use a wrap of masking tape to do it, but that gunks up the launch rod with adhesive, so I never do it myself.
One thing to remember when propping your rocket a couple of inches up off the blast deflector is that you're also shortening the length of the effective launch rod -- if the rocket is marginally stable you might not want to do that, although a difference of a couple inches isn't likely to make any big difference.
Exhaust backwash onto the base of rockets is kind of an occupational hazard, along with ejection charge soot building up inside the body tube.
One thing that helps also is to wipe the rocket down with a damp (not wet) cloth shortly after recovery. While motor exhaust smoke is still fresh, it usually can be wiped clean if your rocket has a fairly smooth paint finish. If you leave it sit for days (or months) the grit usually seeps into the paint finish and is just about impossible to get rid of.
Exhaust grit also gunks up launch rods, making them sticky -- it's usually good to give the launch rod a quick rubdown with steel wool or a alcohol-soaked cloth after every couple of launches. (Don't use alcohol to wipe off the rocket, it will attack the paint job.)