HELP! My supply is drying up...

I'm a working, breastfeeding, FTM who occasionally supplements with formula when my pediatrician suggests to. But lately I've noticed my 4 month old getting really fussy while nursing, and when I used to be able to pump 6 oz total EASILY, just now, I only pumped 2.5 oz after nursing 3 hours ago.

I'm in desperate need of tips, because while, yes, I do complain sometimes about nursing and occasionally the thought to fully formula feed crosses my mind, but I love nursing my daughter and really would like to continue... so what do I do???