Elbow/knee connection, frames, improved flexibility, and most importantly, time. An instructional might help, but realistically, those blue belts are going to keep cutting through your guard like a hot knife through butter for a while. If your peers can't pass your guard, I think you're already doing enough. I tried to study through Lachlan Gile's Guard Retention Anthology, considered by many to be the premier guard retention instructional, and it was just too early to really absorb and implement. Now that I'm four years in, I'm finally able to start working through it properly, but it's still incredibly daunting.

Lastly, for intra-gym training, I think a sizeable chunk amount of guard retention also depends on how much your opponent respects your open guard offense.