I keep a spreadsheet open and when I think of something I feel my audience will profit from, I add the post to my list. If I am away from the office, on my iPhone & iPad, I use the evernote app to write down these pearls of wisdom. In my job I am on the web all day, and things I see spark interest. The synapses get rocking and I come up with a post which will benefit my readers. I add things almost every day and the result is oodles of quality content suitable for social media posts.
Now if you are wondering more about the content of your social media posts, I refer you back to one of my favorite blog posts, The Power of SME (Subject Matter Expert). I always advocate posting things that portray you as a subject matter expert or thought leader.