One technique is, alongside flex, to use Sass/LESS/Stylus mixins (or just CSS classes), using float and widths: IE8/9 use those, they're ignored if flexbox is supported. It takes a little work, and you have to limit yourself somewhat, but it's reasonably robust. Effectively, you describe a float-based grid, but include all necessary flex-based declarations. Alternatively, you describe a display:table based grid, but that has fairly serious downsides.

All current hacks available are pretty fragile. Best solution is a separate set of rules scoped to .no-flexbox (or .flexbox if you're going the other way). It's a massive pain in the arse, tbh.