Matthew Henry’s Method for Prayer has been put online by Ligon Duncan and others. If you have not read it, you should. And now there’s no excuse, it’s free! This is something that is worth returning to again and again to help you in prayer. One bit about buying the book that I would recommend over the online is that it comes with Henry’s work on Communion with God in it as well, which is another work worth returning to over the years.
I just noticed that if you register on the sight you get access to Communion with God as well.