From my BJJ/Submission wrestling training and experience, the instructor taught two good ways of dealing with the mount:
1. Sweep/Roll- This is the best one to do, and is easier if the guy doesn't have good top control. What you want to do is tie in only one of his feet with one of your legs (so, pull one leg up and let him get his "hook" in it), but keep the other one straightened out. Then, roll your body weight with all your strength towards the hooked side, using your free leg if needed .. if you do it right, you might be able to roll around successfully and end up in his guard. The basic idea is that the hooked leg ties him in and keeps him from rolling against you and getting your back, while your free leg allows you to raise your body and get some roll momentum. The better the guy's top control, the harder this is to do.. but it becomes a lot easier against people with a weak mount .. too high or too low, too unbalanced etc. The whole time, protect your head by either keeping your forearms up and blocking his blows while moving your head to dodge as much as you can, or pulling in the guys head to close the distance and keep him from getting a good shot.
2. Shrimping: This is when you try to buck and push the guy's legs lower and lower in the mount, one leg at a time, until you can either wiggle out or get guard. The way to push is to use your elbows or arms against his leg while pulling your waist up past his mount. The problem is that when you use an arm to push, you leave an opening for punches.
3. Give up your back - (This is just applicable in MMA/UFC rules, on the street or a no rules fight, a guy on your back the worst thing to have; he's going to kill you with blows to the back of your head). This is really just a trade off of one bad position for another, but some people are more comfortable in one than the other. If you have good submission defence and are a smaller guy, giving up you back can be an easier way to work yourself out of the mess. You will get hit less due to the rules of no blows to the back of the head, and some guys with with mount control don't have as good back control. If they get too high, you can wiggle your upper body and head through their legs and squeeze out the other side (but beware of submissions, armbars and triangles can be pulled off if you're not careful). You might be able to shake off a hook and roll around into half guard. Stuff like that.
I wish I had some videos or illustrions to link to, but I couldn't find any.