With titles like, “Let Me Put My Love Into You” and “Givin’ the Dog A Bone” it’s not really hard to get the jist of the song.
Suffer the little children to come unto ME!
I used to do this to the Beatles’ “Drive my Car”, but it was less fun when I realized they actually meant most of the innuendo. (heheheh.)
I did something like this when playing Rock Band at a New Years Eve party the other day: the song was Welcome Home by Coheed and Cambria and I sang
Of course, those are the real lyrics, but no one at the party knew that since they bowdlerized them on the screen. So everyone must have thought I was drunk and making shocking stuff up, especially since whatever they put up on the screen actually sort of made sense…
//hey, that maybe explains how Claudio comes up with his nonsensical lyrical stream.
I’m in pretty much the same boat, execpt I do it to songs I like too. It makes me like them more, but it makes karaoke night more difficult when I get my lyrics mixed up with the real ones. 
Anyway, Toby Keith’s “Courtesy of the Red, White, & Blue (The Angry American)” is one of my favorites, mainly because of the visual.
The real lyrics go:
Hey Uncle Sam put your name at the top of his list,
And the Statue of Liberty started shaking her fist.
Mine go:
Hey Uncle Sam put your name at the top of his list,
And the Statue of Liberty started shaking her tits.
Enjoy,
Steven
You don’t want to know my variation on “Blow the Man Down”.
I had a friend in college who made up her own lyrics to R.E.M.'s “Losing My Religion”:
I peed in the corner
She shit in the spotlight
He lost his erection
Well. Consider my purity thoroughly sullied.
Kenny Chesney’s Summertime:
Two bare boobs on the dashboard
Makin’ love to an old whore…
SSG Schwartz
When you change “eyes” to “thighs” it really makes an interesting difference in Peter Gabriels “In Your Eyes.”
Likewise, you can get some good results on many different songs by changing “face” to “ass” or “love” (as a verb) to “tongue.”
Enjoy.