Velcro?
Rare Earth magnets & steel washers on the inside of your vest?
Chicago screws and grommets?
Each way presents it's own benefits and drawbacks. Myself, I went with grommets and chicago screws epoxied inside my armor plates. They have a little give, they do not move or shift out of position permanently, and they go on the exact same way every single time. However, you REALLY need to get the screws set in place right the first time. You also have to punch the corresponding grommet holes in the right spot the first time.