What's the best way to shield a rider from the wind in a group (most aero position in the group)?
When you're in font of the rider that you want to help,  then the more aero you are, the less shelter you give.

Riding in a low body position, head tucked and arms narrow makes it easier to ride at speed, but means that you aren't puching through the wind as much for the rider behind you. 

You'll want to sit as upright as possible, arms a little flared with your head held high to push as much of the wind as possible, with the added bonus of this slowing you down as well. If you really want, you could even unzip the jersey a little to have it flapping in the breeze a bit.

The last thing that can make a huge difference is how you pace the effort. The rider sitting in will generally get more of a draft the faster you ride. This means that when riding the flats or downhills, you can go about as hard as you want, then on the climbs you just ease up to a comfortable pace for him. In a headwind, it is very easy to hold a wheel, whereas a tailwind will make it far easier to drop him.

You also want to absolutely minimise surging when you ride. When you want to accelerate, gradually ramp it up rather than jumping away and putting a gap into him. Things which make it much harder to sit on are weaving about while riding or getting out of the saddle regularly, throwing your bike back. If you hold your line and make a hand gesture before getting out of the saddle, then the rider behind can sit much closer to you safely.