Buy a pair of Fat Gripz asap.
Relatively cheap, will last forever, unbelievably versatile.
After your Farmer's Carry work, do 3-5 sets of static holds with the heaviest barbell you can tolerate for 15-20 seconds.
You can also roll a few sets of Farmer's Carry with the Fat Gripz with lighter implements than you normally use (start around 50% of typical load, back off or step it up accordingly) the same day or a second day of the week.
Also, per Dave Tate, your weak link in grip is the pinkie. Get one of those black spring loaded clips from an office supply store/closet and try to open it with just your thumb and pinkie. Don't be surprised if you can't right away. Work on it.