SEM expert Andy Beal provides this list of tips:

  1. Only buy links from sites that are highly relevant to your web site content.
  2. If the site you are buying links from already has more than 5 paid links on the page, walk away.
  3. If the site labels the links as “sponsored” or “paid links” or anything like that, walk away.
  4. Be selective in your targeting. Don’t buy footer or sidebar links if you can help it. Buy a single link from a relevant page.
  5. Vary your anchor text. Try to make your anchor text look natural. If you buy links on 100 pages, and they all use the same text, you’re asking for trouble.
  6. Avoid any paid link where the seller is also an affiliate for the broker.
  7. Check that the page ranks well for its targeted keywords. If it doesn’t rank well for its own keywords, it will likely not help you.
  8. Point the links at different pages. Don’t buy lots of links for your homepage.
  9. Try to get the links in a contextual format. A link that is part of a highly relevant paragraph will be more valuable.
  10. I guess I should round this out to ten. :-) Don’t worry about PageRank. A brand new page may be highly relevant to your industry and rank well, yet the PR shows 0/10. Ignore that, PR takes forever to catch up.

And I agree with every one of them!

I can add:

11. Check back a few weeks later and make sure they haven’t added a “nofollow” to your link
12. Use the keyword suggestion tool at Google Adwords to find related keywords for the keyword you wish to found for - and use a related keyword in your link text