I tend to Spoonerize -- deliberately. This world is, for example, full of cog soggers and sock cutters.

But I attempt the more creative, true curses:

"May your tires rot along with your privy parts!"
"I hope you get a case of clap for every hair that falls out of your head!"
"May your children grow up to be disappointments to you!"
"May the grass grow tall upon your grave, and soon!"

People riding with me seem surprised and somewhat amused by this sort of thing, heart-felt as it is.

Of course, to do it right you should fast in front of the subject's doorstep until an apology is forthcoming or until you die, whichever comes first.