You can gain credibility and relevance when an unbiased objective source writes about your company (i.e. journalist, 3rd party website, review)
Search engines love press releases more than blogs. Because they appear on the major sites (Yahoo! News, Google News, etc), these releases earn brownie points in consideration to inbound links to your site. Use keywords in your press release headlines when possible.
If possible, make an RSS of your press releases and find a distributor for this feed. If a website finds your releases important enough to share, they will tie into your feed and distribute this content for free!