Simple question, really.
What are the smallest changes you've made to your campaigns to produce the biggest return in profit?
Some of mine:
1. Switching from Player A to Player B, and vice versa.
If the Player A spot on TrafficJunky requires a bid of 0.50, and the Player B is 0.35, and the difference in performance is only tiny, why would you bid on the more expensive placement?
You can sometimes add 20% to your profits just by finding the right placement where value meets performance (usually by deducting competition).
2. Using a native, professional translator.
You don't want to stop at hiring a professional translator and leaving him to get on with it (although for many, it's a good start). You want to hire a native speaker who understands what you are trying to achieve.
I've posted quite a lot about the power of local slang. A few native words can turn a struggling campaign in to a winner. I've seen this yet again in the last week with the term 'Mam' massively outperforming 'Mum' in the Irish market.
3. Avoid cannibalising your profits.
Pressing the Pause button when you need to, and resisting the urge to duplicate your ad in to multiple placements on the same page...
I generally find this makes a big difference to both profitability and sustainability.
4. Implementing jQuery.
The more engaging the Landing Page, the better the conversion rate.
However, some pages that try to engage end up harming the conversion rate by being overly clunky and ultimately losing clicks. jQuery has made a huge difference in allowing me to get the user from A to B, whilst hard-selling him along the way.
Perhaps the most common example of this is the way users are 'qualified' for adult dating offers with a loading bar followed by fadein text. It's a technique that goes way back to the weight loss rebill days when users were asked to 'qualify' for a free trial.
jQuery makes you money.
5. Believable Scarcity
Nothing converts like the genuine fear of missing out on a good thing.
Learning how to turn a token attempt at scarcity in to a believable attempt with plausible explanation is probably one of the most important variables you can be split testing today.
Something I've been wondering for a while: How do you effectively split test Player A vs Player B?
To compare the 2 you ideally would want to run a.) the same ads b.) over the same timeframe. Problem is, this could lead to identical ads being shown in both placements and therefore screw up your data.
On the other hand, if you run Player A on let's say Monday and Player B on Tuesday, your data sets are not really comparable.
You could test one batch in Player A for Week 1, then another batch in Player B for Week 2. Compare the weekly stats and there's your winner.
It's difficult to remove all bias from the testing, but you will pretty quickly get a feel for which campaign is putting more money in your pocket.
Recently I changed a background picture and my lander became hardly readable. Conversion rate and ROI nearly doubled since then 
Great tips - particularly the JQuery one. I'll do a JQuery tutorial soon after I've finished the Step By Step Guide!
@peanut - Definitely, backgrounds on landers make a huge difference. I've seen +50% CVR or more from changing to a background that's similar to the offer background.

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
Dynamically pulling up the image used in the ad, and showing it in the lander. Collecting more monies now.
I've literally changed 1 word on a landing page and gone from 100% ROI to 300% with Mobile. I spent 10hours a day in thesaurus/urbanthesaurus after than trying to tweak it further, but the previous word stayed the winner.
Adding a script that makes the screen jump to the final CTA after the page is done "loading" so the user doesn't have to scroll has worked good for my rules lander type pages.
Also, making the final CTA button on my landers obnoxiously large and blinking has increased CTR significantly.
Definitely! It's incredible how blind people can be to CTAs - even if you think it's already bloody obvious, making it MOAR OBVIOUS still often helps 
Offer page is red. Traffic source page is blue ( facebook ).
What background color will convert better?
Split-test both and see!
Do you guys know of any sample landing pages that use jQuery or more specifically the thing that clubdrock mentioned
"Adding a script that makes the screen jump to the final CTA after the page is done "loading" so the user doesn't have to scroll has worked good for my rules lander type pages."
Could you elaborate on what you mean by this? What could you write while the bar is loading?
Do you have any examples of landing pages that uses jquery? Not that i want to duplicate it but i want to see what you meant by using jquery in landing pages myself.
Here's a truly excellent introduction to JQuery: http://www.codecademy.com/en/tracks/jquery
. Sincerely yours, native Dubliner and Affiliate Manager, Eoin (Owen) Casey