(Another long Ralphie post, here we go)
https://en.wikipedia.org/wiki/Labouch%C3%A8re_system
My lines for example are 10-10-10-10, and all I'm trying to do is win $40 and move to my next line. Your bet is the sum of the outermost numbers, so your first bet is 20. If you win, you cross out the outermost numbers, and your line now looks like this:
10-10
Your bet is 20 again. Let's say you lose. Put that 20 on the end, and your line is now
10-10-20
Your bet is now 30. Go until the line is gone, and you'll win $40 minus juice. Over time, you'll have to make a line just for juice covering, but that's fine.
So after I cross off the whole line, I make another line of 10-10-10-10 and do it again. As your bankroll builds, you can bump it to 10-10-20-10, and go incrementally from there.
I've been doing it for 5 years, and am up nicely. It's too slow for most people, but most people are losing gamblers.
People will try to tell me it's a flawed system and it's based on martingale, so that's bad, but they can never explain how. They just like closing their eyes and parroting what they heard somebody else say. And long term they lose and I don't so there's that.
Lines do get a bit out of control, but if you stay with it, they do go away. You have to have a sizable bankroll to start with so you can absorb these lines, but if you have solid plays, they don't happen that often. Make sure they're as close to -110 as possible.
And I never ever parlay or play futures because it's set up for you to lose horribly. The odds are terrible. Most of your fun bets are.