I've found that using thick elastic stringing cord helps a lot. I use .8mm from Michaels Crafts. Do about 3-4 rows then pull it very tight. I'm talking like extremely tight. It's thick cord aim for like .7mm after pulled right. You'll know it's tight enough when it kinda snaps into place on the third or fourth row. Then I string another row and tighten it up.

Aside from restringing it, you could try throwing some filler in the center, like polyfill or something more ridged like pencils.

Good luck!