Welcome to the Internet.
Short of traveling the world and personally calibrating everyone's monitor, there isn't much you can do.
In the world of print, there are standards and you can process you images to meet those standards so as to have a reasonable assurance that a properly prepared CMYK image will print consistently. Nothing like that exists for web displays. Although, starting with a properly calibrated monitor is important since you will at least know what the image should look like.
I wouldn't just give up. Here is what I do: I test out my site on every machine I can get my hands on. Not only for how images look, but also for how the website itself functions. Every once and awhile, I'll stop in at my local Best Buy and call my site up on every machine I can find, just to see how things display. When I'm traveling and they have an Apple Store, I'll go in and navigate to my website on their iPads, to see what things look like.
Most stores are pretty understanding. If they ask I always explain what I'm doing. Since the salespeople are usually Geeks, they get it and some even help. I figure I'm probably not the craziest customer they've dealt with (although maybe close).
If 90% of your images look decent on 8 out of 10 monitors, that's pretty good. If it's only about half the monitors or half the images, then maybe you need to reconsider your post-processing. As with most things in life, the more you play at the extremes, the greater the risk.
If you want the world to see your images and be impressed by them (and who doesn't) you want to choose images that will display well. People on this site obsess over the margins, but most of that is for the benefit of personal pixel peeping.
The people you want to impress don't give a hoot about shadow or highlight detail. They don't care about subtle variations in color. They only care if the image works. If you find that a particular image doesn't work on a high percentage of monitors, then you have to be ruthless and take it out of your portfolio.