yep, you have to always be adding content, at regular intervals. Or, at the very least, changing or improving content you already have.
yeah check your onpage optimization and content will help. link building is a great help too. but the former one come first.
Haha of course you don't think so, because you sell links. It is VERY possible that some of the paid links (not all by any stretch) have been devalued. Updating content can help, but I have pages that haven't been updated in a long time that remain on the first page. Building one way relevant links can be difficult, which is why people try to take the short cut to pay for links. Open a blog about your subject matter and link to your site, then build a myspace profile with a lot of friends and your subject matter. There are a lot of blog sites and social networking sites that you can do this for. A tried and true method to move up in the ranks is to build a lot of relevant external websites with unique content. Link from these to your main website and they will all rise together. It's much better that way for the long run opposed to buying links or spamming. Also, just say NO to MFA (Made For Adsense) type pages.