Relationships often end. This doesn't usually happen out of the blue, either. Rather, your boyfriend will start acting differently before he actually breaks up with you. While it is not healthy to be over-analytical and constantly look for signs of an impending break-up, you should be aware of a few things to look out for that might indicate he is thinking of leaving you. Bear in mind, though, that there are many other reasons your boyfriend's behaviour might change, so be careful when applying these signs.
If your boyfriend starts to be less considerate than he usually is, this could be a sign that he is going to break up with you. For example, if he seldom breaks plans without a good reason, but then simply doesn't show up when you make a date to meet him, he might be thinking of breaking up with you. The reason for this is because he feels less invested in the relationship, and therefore does not feel as obligated to be considerate. If this is the case, of course, he probably wasn't that nice in the first place, so you're ultimately going to be better off.
Everyone fights, but not over everything. If your man starts a fight over something meaningless, the odds are that something deeper is on his mind. So, if you find yourself in a lot of fights over things that don't really matter (like leaving a cup out or pressing "sleep" instead of "shut down" on his computer) then you should start to think about what else might be on his mind.
There's no "normal" amount for couples to have sex. However, there is probably a normal amount that you and your boyfriend have sex, as relationships do have their routines. If you find yourself having much less sex than you used to, this can be another indication that a break-up is imminent.
As with everything, though, it might not be about you. Ask yourself what else is going on in his life -- has be been working hard on a project and coming home exhausted? Have you been renovating a house together, with him sawing lumber for several hours after work every day? He might just be tired or otherwise occupied, so give him some time and space.
Disconnected body language can also indicate that a break-up is in your near future. Look out for things like feet facing away from you in social situations, him patting you on the back when he hugs you, and generally being less intimate than he usually is. It is easier to break up with someone you're not often intimate with, so he might be cutting down on intimacy to make the break-up easier on both of you.