I have fished as a coangler in some major tournaments and plan on doing so again this year. From my experience, a coangler can often control how they will be treated by a boater; even one with many bad experiences. Some boaters and some people in general are just jerks and there is nothing you can do to change it. However, most of the time the little things count for a lot.
1. Don't be a hassle in the morning. Get yourself to the meeting location ON TIME. Know how to back in a boat and offer to help with launching/trailering.
2. At the Bassmaster Opens, I usually stop for a bagel or something for breakfast, and a sandwich for lunch. I always offer to bring my boater breakfast/lunch and pick up ice for the boat if needed. Usually they say no, but the offer is what counts. If they do say yes, I am happy to bring it.
3. Be respectful of their boat. Don't leave your tackle all over the place. Keep everything neat and organized so they can do their thing without you being in the way all the time. DO NOT cast to the front of the boat unless invited to do so. Respect the imaginary perpendicular line that intersects the middle of the boat. Do not cross it unless specifically allowed by the boater.
4. This one is my own thing: be excited for their catches, and usually they will be excited for yours. By excited, I don't mean flip out Ike-style, but your attitude toward netting their fish and keeping it positive will often make your boater much more ready to net your catches. You aren't on the same team, but you are also not against each other (at least in the bigger tournaments which I am referring to). I am a fan of fishing and I do want to see my boater do well.
5. Offer some kind of money at the end of the day. Do your best to estimate half the gas. After all of the previous steps, most boaters won't take your money anyways, but be willing to offer it! You might get a jerk once in awhile who will run around all day and then take a large estimate from you even though they chose to burn that absurd amount of fuel, but in the long run, acting in the appropriate ways listed above will save you money and make your life and results a lot better!
Anything I'm forgetting?