The 70% that Apple pays to its developers is pretty reasonable when you consider the backend expenses that are handled for you with the App Store, such as Hosting, Bandwidth, and Marketing. Although Apple has a way in which they will give you an extra 5% on your App Downloads, and possibly even more.
The iTunes Affiliate Program is a program that Apple created to allow for anyone with a website to provide links to iTunes content and receive a commission based on the downloads for the traffic that they generate.
If you already have a website for your App, you can join the affiliate program and then modify the existing url on your site to now include your affiliate id, and you will now receive an extra 5% from every download that you send to the App Store from your site.
I set this up for my apps in a matter of minutes, just fill out the Linkshare application, and apply to join the iTunes program. Once you are approved, you will be taken to the link generator for iTunes. From here just find your application and it will generate an html url tag that you can insert directly into you web site and start receiving 5% for all traffic off that link, as well as 5% off any other purchases made within 72 hours.
You will have the option to create a text link, a small image link, or a large image link.
For an indie developer like me this is a great opportunity for some more revenue with really no overhead on my part. Also the opportunity to generate revenue not just for downloads from my app, but all purchases made in the iTunes store within 72 hours of my referral, is even more exciting.