Forget diagramming, try "treeing" sentences. We had to do that in a Linguistics class I took (grad level). You created a "phrase structure tree" in an attempt to derive the "meaning" of a sentence (instead of just reading it and guessing).

For example, you might tree the sentence:

"Holy shit! The place is on fire!"

in order to determine exactly WHAT the speaker meant to convey.

Likewise, the sentence:

"Put up your hands or I'll shoot you down like a dog!"

would, after being treed, give up its meaning. If you'd like to practice this, derive the meaning of the following sentences:

"If you touch me there again I'll break your goddamn arm."
"Let's get the f**k outa here!"
"Sorry about your dog, mister."
"Just shut the hell up, okay?"
"Now, maggot puke, drop and give me fifty and don't you EVER badmouth my Marine Corps again."

Isn't it nice to know that there is a way to find out just exactly what someone means?