You may see a drop in Google Referrals
According to the AdSense blog, Google is currently going through a validation process to make sure referrals are all valid. I haven't noticed a drop in referrals myself, but if you have, this may be why.
We're always working on new ways for you to get quality ad inventory, as well as developing safeguards to ensure that the conversions generated on publisher sites are valid. As you may know, referral ads are paid on a conversion basis; a conversion is considered a specific action defined by the advertiser. That action might be a sale, a clickthrough to a specific page on an advertiser site, or an online sign-up for a promotion.
We've recently made a change to help make sure that the conversions generated by referral ads are valid. For some publishers who display referrals for non-Google products, the earnings you receive for the ad may now be less than the maximum referral value displayed for that ad. This is because our system will initially place a restriction on referral earnings as we monitor click and conversion data to determine that the conversions generated are valid. Once this validation period ends, you'll begin earning the maximum value of the conversions as displayed in your account.
We can't say precisely how long the validation period will last, but will make every effort to ensure that it's as speedy as possible. It will vary by publisher since site activity is different for everyone, and we anticipate that the majority of you will be minimally affected. However, advertisers will only be charged the maximum referral value once a publisher has completed the validation period.
Our goal with this change is to ensure the long-term effectiveness of the referrals program for both publishers and advertisers. By increasing the value and quality of the referral network, we believe we'll attract more advertisers and provide more opportunities for you to earn.
Effective Adsense advice nobody ever talks about
Whenever I read tips about Adsense, people are always regurgitating the same things over and over: placement, blending, etc. Even Adsense ebooks never have anything new to say. I'll never waste your time or mine by posting such tired, played out tips, but I do have one small piece of advice that I've never seen anyone talk about.
If your goal of creating a website is to make money off of Adsense or PPC ads, you need to consider 
your demographic, or who will be visiting your site. For example, let's consider two types of websites. Site 1 will be about Adsense and making money online and Site 2 will be a baking site full of recipes. The likely demographic for Site 1 would be younger people who know about Adsense and can spot an ad a mile away, ironically giving them the label of being ad blind. Site 2's demographic would probably have a wider age range of older people and these people aren't going to be as ad blind as Site 1's audience. Setting aside all other factors like market saturation of each subject, if you were going to market each site equally and get about the same number of visitors, which ones do you think are going to click more ads? The ad blind Adsense-heads or grandma poking around the internet looking for a cookie recipe? Site 2 would definitely have a higher CTR than Site 1.
This is something I've learned from experience with other websites I run. Subjects with wider demographics consistently get better CTR than my blogs with narrower audiences. When I say wider demos, I don't mean more people; I mean wider age range and backgrounds. In fact, one of my higher-earning sites has a fraction of the traffic of one of my blogs, which has lots of traffic yet lower earnings. This is why knowing your audience is so important.
And here are a few things to remember. Always keep your demographics in mind for everything from website accessibility to the method of monetization you use. If you make a website difficult for older people to use, they're not going to be returning. Yet if you make websites too simple, they may seem boring and younger people may not return. Also, I used Adsense as an example, but you need to keep your demo in mind no matter what type of monetization you use. There is no one size fits all solution to monetizing a website so this is one of those things you have to figure these things out for yourself if you're going to be successful.
Hold the ads when launching a new site
Some time ago someone mentioned that when you have a brand new website to promote, you should leave all advertising off of it for a while until you begin to establish solid traffic. I think it was Shoemoney who said this, but I'm not 100%. Anyway, when I read that it gave me a little reassurance that my instincts were right because I've been doing that all along. When some people have a new website, the first thing they do is get all their ads, like Adsense, to look just right. The only problem is, when you go to start marketing it, if the first thing people see is a bunch of ads, they'll figure it's another MFA site and not pay much attention to it.
Here's how I deal with this using PHP... First, when I'm designing the site, I will go ahead and add the space for advertising or throw in the adsense ads so everything looks how I'll want it to when the site is running full-bore. Then I use a simple PHP snippet to hide the ads until I'm ready to show them.
Most sites I have use a config file for database connection stuff. If I don't have a config file, I'll use any file that gets included with every page or make one just for this. Inside that file, I'll put a simple variable like:
$show_ads = 0;
Then, wherever I have ads, I'll use a simple check to see whether they should be displayed or not, like this:
if($show_ads){ //show ads here }
Obviously you would replace "//show ads here" with your ad code or an include or however you show ads. By doing it this way, I can do site design all at once, getting it to look just right, then hide the ads while I market the site. A few weeks or so down the road, I just switch the $show_ads variable to 1 and voila, advertising suddenly appears throughout the entire site.