Using Google Webmaster Tools:
I know you also guessed that it can only be done by Webmaster Tools. When we submit our sitemap Google index all our previous posts but not the new posts. So we use the Google Fetch url feature to done this task.
1.Go to the Google Webmaster Tools Homepage and register yourself and verify your blog.(Skip this if you already have done it)
2.Now get into your site's dashboard.
3.And choose Crawl, Then click on "Fetch as Google".
4.You'll seen page similar to this-
5. Paste your URL of your post in that box,click fetch.
6. Wait until the fetch status shows the "Success" message. (Generally takes 2-3 seconds).
7.When fetched,it shows "Submit to Index" beside the fetch status. Click on that and wait until you get this "URL Submitted to Index".
8.Its done,Google has indexed your post,try putting the URL in the Google search or just put the post title and you'll see your post like this:
It takes about 40-60 seconds to this,its really easy but productive. Don't have a Google Webmaster account?? Go get it now!
I hope you can easily understand this post and knew that how you can index your new posts in google instantly. If you face any difficulty or you have any query regarding this post feel free to ask till then Peace, Blessings and Happy Indexing.