I rarely find it necessary to signal apology. For thanks, I do the quick wave of the hand… though I will sometimes do it sarcastically or not at all, when some idiot has just decided to confuse everyone by surrendering their right of way to do their “good deed” for the day. I hate these people. They’re gonna get someone killed.
For Thank You: Either a high five or I’ll throw a kiss depending on the sex of the gracious driver.
For I’m Sorry: Either a wince and a shrug or a :SMACK:, depending on the seriousness of the offense.
For: What an asshole!: A “sheesh!” and a headshake.
CAUTION: BEFORE USING ANY “NEGATIVE” SUGGESTIONS, BE THEY HAND GESTURES, HORN BLOWING, OR ANY TYPE OF AGGRESSIVE BEHAVIOR TAKE A GOOD LOOK AT THE INTENDED RECIPIENT. IF THE OTHER DRIVER APPEARS EXTREMELY AGITATED JUST LET IT GO! GET THE HELL OUT OF HIS WAY! TOO MANY PEOPLE HAVE ALREADY LOST THEIR LIVES OVER SOME SILLY ASS PERCEIVED “INSULT” ON THE ROAD.
For thank you I extend both arms above my head.
For sorry I use different signals according to my faux pas.
If I have screamed obscenities I chop the back of my left hand with the right hand above my head.
If I have wandered into another lane I put my hands on my hips.
If they were trying to evertake and I sped up to prevent them I signal with two open hands extended in front of my chest.
If I have given the finger I grab the wrist of my right hand with my left hand.
If I move to go when it is their turn I rotate my arms in front of my chest.
If I fail to stop at a stop sign I swing my arm from in front of my chest out to the side.
If I am holding up traffic I extend my folded arms in front of my chest.
If I deliberately knock over a cyclist I make a chopping motion toward the ground with both hands.
That was you Huh? Grrrrrrr
When passing a vehicle driven by don’t ask all passengers in my car are required to drop their trousers and moon him while singing the bus driver’s hymn. 
…You mean, there’s more than one gesture? 
To apologize, I throw up my hands (at least one of them, since I do still have to steer), make a sad face, and mouth “sorry”.
For thank you, I wave enthusiastically, smile, and mouth “thank you.”
For “you go ahead,” I use the tried and true flash of the headlights.
I also use “Can I go? You mean me?” if I think someone’s trying to be courteous to me, but I’m not sure - it involves pointing at myself, looking surprised, flailing in the direction I want to go, and nodding like a maniac. If they answer affirmatively, I proceed with the “thank you” sign.
Sometimes I think other drivers might suspect me of having a seizure at the wheel.