Having lots of good content is great - but if it is old it's not going to look like your website is much of a priority to you. The search engines actually track and monitor how frequently your website changes - the more often it changes the better your search engine ranking.
What constitutes change?
Pretty much anything - latest news, articles by you or someone else (referenced of course), FAQs added, clients added - which is why adding frequent news articles to your website usually produces good results.
#1 Hint: Tackle one web page a month (use the navigation menu) and ensure that that page is the best it can be at that time - spelling, text, content, grammar and additions. Next month move onto the next page on the list - some months it may mean a thorough overhaul of that page, others a glance through and you are happy with that page. Once you have finished the list, return to the top and start the process all over. At the minimum, this will ensure your website pages are all up to date.
#2 Hint: Don't upload and list your newsletters by date - no-one will go through these looking to see what they might find. INSTEAD use the articles or information in them to create pages of 'articles' or 'tips' that can be found by their title. Load your latest newsletter by all means but allocate the information within the older newsletters (the 'useful to your reader' info) into another area of your website. And a list of text links within your website with relevant keyword text is useful for your search engine optimisation.