So, someone commented on not knowing exactly what a long-term plan entails, and how to go about creating one, so I'll go into a bit of detail. I'm certainly not the most experienced here, but I'll do my best to explain, and give an example of such a plan.
For the sake of this example, we'll pretend we're Turkey. At the beginning of the game, when making a long-term plan, assume for the sake of argument that you simply won't have allies. This original plan will of course change, but it will give you an idea of what you need to accomplish, who you need to ally, and who you need to back stab.
So as Turkey, the nearest 18 Centers are your home Centers, the Balkans, Italy, Tunis, Austria, and Russia. So, if you created no neighborly allies over the course of the game, those are the Centers you must have to win.
Now, lets assume that for whatever reason you decide on a longterm alliance with Italy. Assuming Italy takes, say Trieste and Greece, you need:
Home Centers, Bulgaria, Serbia, Rumania, Budapest, Vienna, Sevastople, Moscow, Warsaw, St. Petersburg... and this is twelve so far. So, you're going to have to expand your long-term plan to other regions. The nearest 6 would most likely be Munich (if your ally Italy hasn't taken it), Berlin, Kiel, Sweden, Norway and Holland or Edinburgh. As you march into Germany however, and 15, 16, 17 centers, you might consider a swift stab against Italy, taking that juicy Greece and Trieste for your final centers.
This of course bars Diplomatic relations. But this is mostly about basic unit-oriented strategy. It's like mapping your path through the country on a road trip. You can see the nearest sights, or you can go a little farther and see others more abroad, but miss out on some that are closer to home.
So that's basically the how things should go in your mind. Closest or easiest centers for you to grab based on your diplomatic relations. If you're intent on remaining allies with a player, factor that in. Otherwise, just be sure you stab at the right moment.