Posters give a number of ways to address the weakness:
I feel like my low back endurance, by which I guess I mean ability to hold a good strong arch, and good form, is keeping me from doing as many sets or as many exercises as I would like. I'm good for some high intensity sets, but just so much and then my back says no more and tightens up.
I've found Bent Over Rows excellent for strengthening the arch. Keep the weight light at first and really flex that arch.
It's popular to be afraid of deadlifting very frequently these days. Your lower back and grip are capable of a lot more work capacity than you probably think. Just keep adding sets until it really starts to bother you. Then cut back and build it up again even more.
Pullthroughs, adding a lower weight, high rep set after normal deadlifts, coreblasters, etc. Lots of ways to do it.
Being that some types of deadlifting (reg. BLDL's, squats, bentover rows etc.) train the low back only in a static way more or less, I find upping the volume/reps on say hypers or lighter RDLS or SLDL's, pull throughs perhaps, some round backing rack work, trains my low back musculature in a flexion/contraction way instead of just training it to stay tight or in arched position.