--Use your turn signal. Every time.
--You do not need to slow to nearly a stop to round a corner. If you do, you need to drive a smaller car you can control.
--If you cannot park it, buy something smaller.
--When at a light in traffic, pay attention. Nobody behind you wants to get stuck at the light because you were like molasses in January when it turned green and took forever to go through. Everybody wants to get through this light; do your part and GO when it's time. (Green = Gas = Go; notice how they all start with G; this should be a hint.)
--If the person in front of you is going the same speed as the car in front of them, no amount of honking will make them go faster; physics usually dictates that matter can't share space and they're probably not willing to run up the other guy's rear bumper on your behalf.
--Pay attention to what's going on around you and more than ten feet in front of your car; if the cars in front of you have been slowing for the last half mile (or you see the light turning red way up there), don't zoom up behind them and then slam on the brakes. Also, your car does slow down ahead of time (this is also physics); maybe ease up on the gas to naturally slow down before you get there.
--Honking and flashing your lights is as rude as walking up behind someone and yelling "MOVE!"
--Conversely, it's perfectly okay to actually go the speed limit in most cases (assuming road and weather conditions permit); try it some time.
--Trying to intimidate other drivers by riding their rear or honking could backfire on you one day; you'll either meet someone with a hot temper and a firearm, or a driver so timid they'll freak out and cause a crash.
--If your lane is ending and I (or someone else) is letting you in, GET OVER. There is no law that says you have to hug that stupid little line and ride it over until it's gone. Get over immediately. I'm not riding my brake waiting for you forever.
--If we are on a freeway on-ramp, STEP ON IT. In a few feet we need to be going 55-75 mph to merge with traffic; you going 20-30 mph doesn't cut it.
--When merging, it's a cinch: car already in the lane goes, car merging goes, car already in the lane goes, car merging goes. Wait your turn or let someone in, depending on which lane you're in. Other merging advice, which goes back to "pay attention to what's going on around you": if we're in adjacent lanes, and you want to get into my lane and my blinker is on for your lane, let's be smart about it and both change when we have room, and then it works... otherwise one person has to get over first and there's this awkward jam until the second one clues in and goes over.