I've been helping a friend optimise his software archive site SoftTester. The site is nearly 5 years old and has about 100,000 pages as well as being listed in DMOZ. Over the last couple of years his site had been slowly losing visitors. By June he was down to only a few hundred a day. Needless to say, his income from Adsense had fallen away to almost nothing.
In June, we decided to do some SEO on the site. We mainly concentrated on on-page SEO and improved page titles and descriptions as well as adding good h1 and h2 tags. His site is database-driven, with most of the content coming from PAD files submitted by software authors.
We changed some of the data used to display info as well as shuffling the position of some the displayed items. Whatever we did, it seems to have paid off. Within a couple of weeks, search engines started sending more traffic to the site. In particular traffic from Google began to grow steadily.
As well as on-page optimisation, we set about getting new links to the site. One of the main ways software download sites get links is by reviewing and making awards to listed software packages. Software authors can then use a nice award graphic on their own websites and link back to the archive. The existing graphics were a bit tired, so I encouraged my friend to buy classy new ones and before long he began to get extra links to his site.
After waiting 4 or 5 months, the number of visitors and page views had grown by a factor of nearly 5 and the income from Adsense had grown along with the traffic. Not a bad result for a few hours work spread over a few days.