A good starting point is to ask yourself why are you blogging? It should not these days be just about key words, but providing quality and relevant content to your readers, not just to keep search engines happy.
Next, look at the sheer number of posts you have, all they all still relevant, if Google is crawling hundreds of pages to find a few good pieces you are probably doing yourself no favours, in fact Google longs things like this and may actually spend lest time crawling your site in the future if it feels it is just wading through low quality irrelevant content.
Removing old or irrelevant content will enhance your brand, to both the search engines and your visitors, a focus on relevant well written content is more likely to establish you as an "authority" something Google seems to look at favourably.
So how to go about it once you decide.
Firstly make a list, table, whatever of what posts you actually have, the URL's and the keywords they are targeting. Next be honest with yourself, which is not always easy, and ask yourself are they actually good, are they still relevant, is the subject duplicated in previous pots, and most importantly if you decide to keep the post, can it be improved? The occasional reread of what you have written is not just an exercise in vanity, you can often spot mistakes, or things you could have done better. If so improve, correct, and enhance.
For the rest of the content, simply delete it! Yes I know it's hard but just do it.
It may take Google a month or two to register the changes, but doing this regularly will reap a reward, both from a search engine perspective and will make your blog more attractive to clients both existing and potential.