I've had a plasma for years & never had an issue. But I also did a lot of research, found out about image retention, burn in, etc. before I bought it & I'm also a pretty anal videophile & calibrated my tv immediately after pulling it out of the box. Vivid, Game mode, or any other factory settings always look absolutely awful!
But yeah, I'd just leave it on a signal that fills the entire screen & has NOTHING that remains perfectly still (such as a channel logo). it should clear it off eventually.
That said, you will still probably have image retention on occasion. For example, after watching an old movie that's 4x3 I can almost always see the sidebars being retained. But then I put it on a 16x9 source & they clear off within 2 minutes or less.
I've never had permanent burn in.
But properly calibrating your tv is the best thing you can possibly do to keep this from happening.