Tags are truly important for each blog post you publish. To a certain extent, tags could be used to replace searching, if done well.
The tags that you may use for your blog posts should be directly or indirectly related to what your post is about. Generally, it should answer the questions: where, what, why, when, how.
For example, if your publish a blog post is about SEO tools, you may consider the following tag options:
Where: website, blog
What: SEO software, SEO tools, internet marketing tools
Why: website optimization, website promotion
When: Panda update, Penguin update
How: content optimization, building links
Ideally, you should focus on 3-5 tags per post, so there's no need to use all of them at once. Just choose the tags that best describe your specific posts.