My husband and I both had lots of bloody noses as kids, so our son has them from both sides! At 13, he still gets them, although not as often.
Dryness is a common culprit and allergies can lead to them as well. Even mild bumps to the nose in sports can get them going. And, truth be told, picking at the nose can cause them as well. Sometimes our son's nose just doesn't stop! It bleeds and bleeds and bleeds for as long as 20-30+ minutes sometimes.
However, ever since our doctor recommended the following routine, our son's nose has been much better. This routine has helped greatly.
When our son's nose starts bleeding, he is to pinch his nose so that no blood comes out. He is then to hold it like that for FIVE minutes--have a clock or watch available, no peaking, no leakage allowed. It feels like forever when you first start trying this, but it's amazing. Even though it's ever-so-tempting to check if it has stopped, you need to remind all present about NO PEAKING.
After the five minutes, then check that it has stopped. It likely has. The next part may gross you out but it really helps to prevent future ones. You need to then pull out the clot. It will be VERY LONG and STRINGY. Keep pulling slowly till it is all pulled out. (Our son thinks this part is very gross and very cool. Me too, actually. It's amazing how long the clot can be.)
The point of doing this last step is to remove the clot that would otherwise harden and be a potential source for future aggravation. If you follow this process, it is likely you will not have a lot of repeat performances.
Like other responders, preventive measures include the Q-tip with Vaseline in both nostrils and a humidifier. When the heat first gets turned on, we start our full console humidifier down in our dining room (making sure it is clean). It basically works for the whole house and we are diligent about filling it. Those two things, along with checking for allergies, can go a long way to prevent them. And, if it all else fails, stop the nosebleed with the trick our doctor taught us. It works like a charm.
Our son is so used to nosebleeds. He has been relaxed with them for years because they are such a part of his life. Calm attention to them can go a long way to curbing them. In early years, they would upset him which would just make them bleed more. So a calm approach will do wonders.
Good luck!