There is only one real answer, absent any off smells or flavors and that's too many fermentables. Infections can cause gushers too, but there would be other signs.
You are either adding too much priming sugar (corn sugar, DME, what have you) or you are not letting your beers reach terminal gravity, which is the point when they go dormant due to lack of fermentables. You need to be using your hydrometer to get a starting gravity and then use it a few more times to be sure that the beer is done. I usually call it good when the gravity remains steady without dropping for 48 hours.
There is no "right number" for determining the final gravity of the beer. That is determined by the effectiveness of the yeast, the style of the beer and a myriad of other factors. Most recipes do include a final gravity number, so you can know what you are shooting for. Unless its a big beer, I usually wait until the gravity is under 1.015 before bottling for a standard ale yeast.
If you are getting uneven carbonation, meaning some bottles that explode or gush, or some that are under-carbed, then you are not mixing the priming sugar in thoroughly enough in the bottling bucket. I created an entire batch that was either flat or near bottle-bomb level by not carefully stirring in my priming sugar and making sure it was evenly distributed.