Q ( paraphrased )
I am working on doing crunches every day and getting sharp pain in my lower back, mostly on an decline crunch.
A- More than likely your lower back is experienceing the pain from lack of support and years of gym teachers telling us the wrong ways to do crunches / sit ups. First thing you should do is get a balance / pt ball - they are fairly cheap and will help get rid of back pain, and work your abs harder than a normal crunch.
SEcondly are you doing any exercises for your lower back? That too could be the problem. basic reverse crunches may help strengthen the spine and those core muscles in your lower back to help.
Do you get the pain any other time? If so I would consult a chiropractor, it could be something else.