Some of my dad's favorites:
Worthless as tits on a boar hog.
I'll be John Brown.
That don't make a hill of beans.

When we're talking and I say, "I see", he'll say "I see said the blind man as he picked up his hammer and saw."

If I made a wrong assumption he'll give the classic line about making an ass out of you and me or he'll say, "well you thought like Caesar."