The best way to reliably gain SR is to simply get better at the game, but there are some other tricks that can help. These tricks will either improve your chances of winning, manipulate the SR system or speed the ranking process up.
One of the best ways to improve your SR is to one trick, we have an article on this here. But to summarize it one-tricking improves your SR because it's better to be a master of one hero than a platnium of all of them and the ranking system benefits high stats relative to other people who played the same hero, so by mastering one hero your stats will be much higher on them relative to other players.
Another way to gain SR is to try manipulating the ranking system's internal MMR, this internal MMR determines how much SR you win or lose in a game and it's based off your win rates and game stats. You can control your game stats, so if you can try to do trash damage/heals/res or anything to get fire just to bump those stats up. Don't ever let this get in the way of winning though, as that's much more important.
If you feel you're far below your deserved rank and aren't bothered to play the 100s of hours required to rank up, you could always buy a new account and redo the placements. This shouldn't work if Overwatch's Ranking system is accurate, but in many cases it isn't. It can take quite a while to rank up especially since the removal of win/loss streaks. I'd only recommend this option if you're sure that you're far below your deserved rank though, because otherwise you're just going to waste $40.