By minimizing the amount of time your eyes need to spend performing accommodation (near-focusing). This doesn't mean avoiding close-range activities like reading or using a computer, but it may mean using a more appropriate lens in such situations (or none at all).
With your prescription when not wearing your glasses your focal range (the distance at which you can see things clearly) will have a limit of around 28cm (1 metre divided by the number of uncorrected negative diopters), meaning you can read & write fine without glasses. Using a computer would likely require negative lenses, but not ones that are at your full prescription. You would be forcing your eyes to perform unnecessary accommodation using those when on a computer, thereby encouraging your eyes to become more myopic.
[see: http://www.myopia.org/ebook/08chapter3.htm ]
You could benefit from wearing a reduced prescription for use on a computer. One that puts your focal limit at about the same distance as the screen to avoid the need for accommodation. Say the screen was 50cm away, a pair of glasses that under-correct by 2 diopters would put your focal limit at that distance. This doesn't have to be an expensive option, btw. There are some very cheap online glasses retailers. Check out zennioptical.com or goggles4u.com. I have bought glasses from both of those in the past and they were fine. I am actually using a reduced strength pair myself while typing this, that i got from Zenni.
When you say you have double vision, do mean an astigmatism, or do you mean your eyes turn in (or out)? If it's your eyes turning in, then I guess that 2nd pair of numbers are base-out prism diopters, in which case you should be able to do without any prism correction when your eyes converge at 50cm. So if you wanna go this route, I'd recommend you take 2 diopters off your prescription, i.e. 1.75, 1.25 (make sure you get them the right way round!), and skip any base-out prism. If it's any other kind of prism, you will need the prism. If you have an astigmatism (the thing that has an associated angle) then I suggest you take around 0.50/0.75 off that (the CYL #) too (but leave the angle unchanged!).
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There is a myriad of evidence out there that suggests that myopia is environment/behaviour-related and not merely genetic...
-The highest rates of myopia in the world are found in major Far-East cities like Singapore. This is thought to be because children there tend to be more studious and spend a lot of time indoors so they are rarely focusing into the distance and their eyes lack exposure to sunlight. -http://www.bbc.co.uk/news/health-17942181
-The lowest incidence of myopia in the western world is found in Australia where the kids play outside a lot.
-A study in Israel showed that children in strict religious schools where a lot of reading must be done have higher rates of myopia than those in the regular state schools.
-And myopia incidence amongst eskimo children hit 80% once they started going to school despite their parents/grandparents having very low rates of myopia.
-Myopia has been shown to progress further amongst law students at a time in their life when it generally levels off for the rest of us, again due to all the reading they have to do: http://www.aaojournal.org/article/S0161-6420(02)01012-6/abstract
-Also, scientists are able to induce myopia in young monkeys by forcing them to view the world through negative lenses: http://www.ncbi.nlm.nih.gov/pubmed/7585177
@Britter: You are probably right about the prism being vertical, in which case it must be kept. I never said take off sphere to correct cylinder. I mean take about half a diopter off the cylinder as well to leave room for improvement. I am talking about a pair of glasses for computer use here, as I use myself. I still drive with a full prescription!
It's a combination of environment & genetics. The main cause is study. Staring at books a foot in front of your face (without reading glasses) does it, if you are genetically predisposed.
You are the one who is completely wrong about the monkey study. I quote: "Both positive and negative lenses produced compensating ocular growth that reduced the lens-induced refractive errors". That means negative lenses inducing myopia so that the monkeys see more clearly with them.
Here is another study where myopia was induced in tree shrews by fitting negative lenses to them: http://www.iovs.org/content/53/3/1593.full
Intermittant use of positive lenses reduced the rate of myopia progression.