1. Rhetorical questions. This is where you ask the audience a question that is meant to make them ponder, to elicit thought or feelings, and to answer in a way that supports your argument. Henry's speech has many, many rhetorical questions. I'll list a couple below:
"Shall we gather strength by irresolution and inaction?...Are fleets and armies necessary to a work of love and reconciliation?"
He asks these questions hoping to elicit strong emotions in the audience, that work them in favor of his argument.
2. Parallelism. This is a bit hard to explain, but parallelism is where you have a very balanced sentence structure with equal parts. You have a series of sentences that are all written in the same way, with the structure in the same order. This helps you to sound planned, prepared, poetic, rhythmic, and helps the audience remember what you've said. The best example of this:
"We have petitioned; we have remonstrated; we have supplicated; we have prostrated ourselves before the throne, and have implored its interposition to arrest the tyrannical hands of the ministry and Parliament."
Note how the sentence structures are similar--we have _____; we have ______; we have ________. Henry just fills in the blanks. That is parallelism, and makes his speech sound more professional, and hence is more persuasive.
3. Logical and ethical arguments. This is where Henry uses logic (facts) and ethics (human rights, and universal truths of right and wrong) to make a point. For example, for logic, he argues that if Britain is so intent on making peace with America, why are there so many British troops in our streets? That's a logical argument. Here's a quote:
"what means this martial array, if its purpose be not to force us to submission? Can gentlemen assign any other possible motive for it? Has Great Britain any enemy, in this quarter of the world, to call for all this accumulation of navies and armies? No, sir, she has none."
He also uses ethical arguments. Consider his last line, "but as for me, give me liberty, or give me death!" which appeals to the ethical argument that freedom is worth pursuing, even at the cost of a life.
I hope that those examples help a bit; good luck!
Patrick Henry uses the following devices in his "Speech to the Virginia Convention":
1. Rhetorical questions. e.g. "I ask gentlemen, sir, what means this martial array, if its purpose be not to force us to submission?" "Has Great Britain any enemy, in this quarter of the world, to call for all this accumulation of navies and armies? No, sir she has none." "Why stand we here idle?"
2. Emotional appeals. e.g."Suffer not yourselves to be betrayed with a kiss." (This is also a Biblical allusion, and as such appeals to one's emotional faith.) "Three millions of people, armed in the holy cause of liberty, and in such a country as that which we possess, are invincible by any force which our enemy can send against us." "There is no retreat, but in submission and slavery! Our chains are forged! Their clanking may be heard on the plains of Boston!" "I know not what course others may take; but as for me, give me liberty, or give me death!"
3. Logical appeals. e.g. Our petitions have been slighted; our remonstrances have produced additional violence and insult; our supplications have been disregarded; and we have been spurned, with contempt, from the foot of the throne. In vain, after these things, may we indulge the fond hope of peace and reconciliation." "It is in vain, sir, to extenuate the matter."
It also may help to hear the speech read aloud, here is a recording: