Bettas and other surface-eaters are especially prone to jumping out of tanks. They've evolved in an environment where jumping is beneficial, if not vital, to survival.
In an aquarium, fish will jump because they are stressed from having too many other fish in the tank, the tank is too small, the water quality is poor, or (especially with bettas) because they think jumping will bring them closer to a mate or intruder.
As with all things related to fish, water quality is the most important factor in determining health and wellness. If the fish is jumping, change the water immediately and take a look at your tank to determine if it's just too crowded. Thin it out by removing some fish if needed.
If your fish is still jumping, the next option is to cover the tank or bowl. With aquarium tanks, you can get an inexpensive tank cover that will have a gap for filters and heater cords, at the pet store, or ask around to any other aquarium owners- chances are, someone has one they don't mind getting rid of.
With bettas, though, you probably keep the fish in a bowl or other small container, which didn't come with a pre-made cover. You'll need a do-it-yourself option!
To make your own bowl cover, go to the fabric or craft store and walk into the bridal section. Find the "tulle." This is a mesh-like fabric that is used primarily in bridal veils and ballerina costumes. A yard of tulle should be very inexpensive- less than $3.
If you're especially masculine and having a hard time bringing yourself into a fabric store, head over to the hunting and fishing store and buy a mosquito net. It'll cost you five times what you'll pay in the craft store, but your machismo will be in tact.
Measure across the opening of the bowl to get its diameter.
Cut the tulle or mesh into a circle about twice the diameter of the opening of the bowl. A quick tip for making a square piece of fabric into a circle: fold it in fourths, then cut a quarter-circle around the edge. This also gives you a decorative scalloped edge, if you're not too precise.
In the center of the tulle circle, cut a small hole, about 1/2 inch across. This is so you can feed the fish without removing the tulle.
Next, get a long piece of ribbon, yarn, or string (yes, you macho men- you can use paracord). Remember the diameter of the bowl? Divide it in half, then multiply it by itself. Now multiply by 4. That's how much string you need.
Hold the tulle over the bowl opening and tie the string around the lip of the opening.
Note: not all fish bowls have a "lip." If your bowl doesn't have a lip, I strongly recommend getting another bowl, as it will be nigh impossible to cover adequately without using tape. Since tape leaves a residue on glass, you are better off getting another bowl, or using the lidless methods described below.
Of course, some people just don't want a lid on their tanks. Lids are beneficial not only because they prevent fish from jumping out, but also because they prevent dust and dirt and kids' toys from falling in. More importantly, lids regulate your water temperature and help prevent evaporation. Even the simple tulle lid helps protect the tank from these hazards!
If you really don't want a lid, or your fish jumps anyway and bonks himself on the head, consider filling your tank only halfway, so fish can jump, but not out. If you do this, respect the fact that your 10-gallon tank is only five gallons now, and change the water more frequently. Additionally, floating plants like Water Sprite and Duckweed are very beneficial in preventing evaporation and in helping your fish feel more protected. If you have floating plants, however, you should also consider a lamp, which means a tank cover anyway (because bare light bulbs next to water is a quick recipe to electrical fires).
After all your hard work, a happy fish may still jump, particularly if it's a surface-dwelling fish like a betta. Just remember that the fish isn't trying to commit suicide- jumping out for them is a bit like diving into the deep end of a pool is for us. Fun for a second, but they sure don't want to stay there for more than a minute!