Well, getting anxious about your gas consumption isn't going to help your gas consumption!
We all have to breathe a certain volume of air per minute through the gas exchange portion of the lungs. How MUCH we have to breathe is determined by our rate of carbon dioxide production, because the carbon dioxide level in the blood (which the body keeps VERY tightly controlled, because it affects the acid/base balance) is directly related to the volume of air you move through the air sacs, or alveoli.
But not all the air we breathe gets to the alveoli. If you think about how the lungs are constructed, you have the trachea, which branches into the two major bronchi, which then branch repeatedly out into the lung tissue itself. None of the actual passages participate in gas exchange, so air has to get pretty deep into the lungs to begin to do that. If you are using a rapid, shallow breathing pattern, then the majority of the air that you are sucking out of the tank is just washing back and forth in the bronchi -- effectively, this is wasted. That's why we teach a slower, and deeper than normal breathing pattern for scuba -- it's the most efficient way to use the gas you take out of the tank. Anxiety makes us adopt a rapid, shallow breathing pattern, so anxious people are inefficient with their gas. Overexertion can make you adopt that kind of pattern, too.
And exertion is the other half of the equation. The more muscle effort you use, the more CO2 you make, and the more you have to breathe. So the quieter you can become, the longer your tank will last. New divers are often unstable, and move a LOT -- they wave their hands and they kick constantly, and they go through their gas very quickly. As you learn to balance your equipment, so that you aren't feeling unstable, and as you become better at establishing and maintaining neutral buoyancy, you can reduce those now unnecessary movements.
One of the most pernicious patterns is diving head up and feet down. As you can imagine, if you are tilted head up, then when you kick, your kick is driving you UP in the water column. Since you don't want to ascend, you have to provide a force to counteract the one coming from your fins, and the way you do that is to dive negative all the time. What this means is that you swim through the water, expending energy which results in no net displacement of the diver . . . but it still requires gas!
So, in sum, the way to reduce your gas consumption is first of all to relax about it; it is what it is, and stressing won't make it better. Work on adjusting your equipment so that the tank is secure on your back, and move weight around until you can float in a horizontal position. Swim horizontally, and stay neutral, unless you want to change depth. Try to extinguish unnecessary motion, like sculling while hovering, or waving your arms. Keep a slow, rhythmic breathing pattern -- slightly slower and slightly deeper than what you would use sitting quietly on your couch (kind of like yoga breathing).
And the last bit is that, no matter how efficient you become, how much gas you use will be to some degree determined by who you are. A 6'6", heavily muscled man WILL use more gas than a 5'4" woman, no matter how experienced or efficient he is. I'm sure you can improve the 20 minutes on a 130 situation, though!
