These days I mostly clamp and pump for girth, but when I jelq with the intent of increasing girth I usually do it with two hands: one hand with an underhand grip beginning around the base, and the other hand holding the glans and pulling outward to sort of stabilize and guide the stroke. The hand holding the glans has the head in a sort of claw grip, with the index and second fingers on the underside of the glans, and the thumb around the top, just under the coronal ridge so the glans are cupped inside my palm.
I also do horse jelqs like VVC describes here, where I jelq a semi-erect cock while compressing the shaft with the hand that’s holding my glans.
When I’m jelqing to flush my cock of old blood and lymphatic fluid — like in between pumping or clamping sessions — I usually do it overhand. I switch hands too, because I find my thumb can put more pressure on the sides of my cock than my index finger can.
(Pictures of different grips: Dry Jelq - Step by Step)
I also often jelq while sitting on the edge of my chair and pull hard downward towards the floor as I jelq, so I get a good stretch with each stroke.
One grip to try: a sort of two-handed vise grip. Wrap one hand around the base with an underhand grip, then take your other hand, in an overhand grip, and wrap it around the first hand, gripping tightly. You’ll find that with the combined strength of both hands you can get a very tight squeeze. That’s also a good way to do tight uli #3 squeezes with a semi- to full-erect cock. ‘Not for newbies.