Of course there is always the contrary author who doesn't like to Follow The Rules. But for the majority of YA books written, there is a clear way to determine how a Love Triangle is going to progress, and who is going to end up with whom. This information works best with the Paranormal Romance genre, but is applicable to essentially all speculative fiction - dystopian, fantasy, sci-fi, etc. Contemporaries are less predictable, but generally they don't drag out Love Triangles to the degree of multi-book series.
I'm also aware that not all Love Triangles involve two guys and one girl, and some progress very differently. There are even reverse love triangles, and other love shapes. If you want to know about love squares and other geometric shapes, visit Jen @ YA Romantic's awesome post on The New YA Love Geometry. (It includes graphs!) Obviously, you'll have to adapt these rules to those situations, accordingly.
I will use the following key:
Heroine - the person at the point of the triangle
First Love - the person the heroine falls for first after the book begins
Other Guy - the second person in the love triangle
Rule #1. The rule of firsts - In 95% of Love Triangle series you read, the Heroine will end up with her First Love, i.e. the first guy she falls* in love with once the book begins, which is most often also her first love. You will avoid most Love Triangle confusion by remembering this rule.
*The falling in love part is important. If for some reason the Heroine is already in a relationship with someone when the book begins, don't get too attached. He is not likely to last for long. However, if the Heroine is already in love with - but not dating - her best guy friend when the story starts, he has more of a chance of sticking around.
Rule #2 If you are reading a multi-book series and the first book ends happily between the Heroine and her First Love, expect that they will be torn apart in some way in the second book. Authors must keep up the tension! Sometimes that means distance or misunderstanding, or a love triangle. The difficult problem for authors is adding in these elements and making them feel necessary and not like filler. (This is actually a general rule for multi-book series, but can be added to specifically fit Love Triangles).
Rule #3 If you are reading a three book series and the first book ends happily between the Heroine and her First Love, but there is an interested third party, you can be fairly certain that the interested person will become the Other Guy in the Love Triangle. Learn to spot these potential interlopers, but remember rule #1 and don't get too attached to the Other Guy.
Rule #4 If you are reading a three book Love Triangle series and the Heroine ends up firmly with the Other Guy by the end of the second book, expect that everything will be reversed in the final story. This almost certainly means that the Heroine will end up back with her First Love by the conclusion of the series. This is when it is especially important not to forget Rule #1.
Rule #5 If a Love Triangle enters a story because it looks like there is some HUGE reason that the Heroine and her First Love can't be together, don't panic. This technique is being used to throw you off. Everything will end happily for the Heroine and her First Love by the conclusion of the series.
I know what you're thinking: If there are exceptions to the rules, maybe the book I'm reading is going to be the time that everything is different. I still wonder that for many books I read, and it's certainly possible. But it is also rare, so don't pin your hopes (or fears) on that unlikely scenario.
Of course I have tons of examples of books that fit each rule, and even a few exceptions. But unfortunately, sharing them would be a spoiler.
Any more rules you know of?