The most helpful thing I have found for deciding whether to use por or para is a graphic on the DR Lemon web site. It is like a four-spoked wheel, with each "spoke" representing various uses of por, and the terminus of the spoke into the wheel representing para. It has helped me visualize and decide whether something is an end result (para) or a path toward a result (por.) I am slowly beginning to feel more confident in which to select.
As for the fact that not every English preposition has a corresponding Spanish one, and vice versa, I think as Robert said, it comes down to memorization. Choosing the wrong one in conversation is frustrating to me until I remember that a lot of my native Spanish-speaking friends choose the wrong words when speaking English. I can still understand them and I appreciate that navigating a language that you have not spoken your entire life is challenging, so I don't think less of them for their errors. I assume they give me the same latitude.