Aiding - Clip gear as high as you can
When leading on aid, standard practice to get maximum reach is to try to clip gear as high as you can.
Let's start with a bolt. Typically, you clip your ladder directly to the bolt is shown below on the right, as this gives you maximum reach. If you clip a quickdraw first, and then clip your ladder to the top carabiner of the quick draw, you're losing a few inches. Pretty much all modern both hangers have enough room for two carabiners, that's why this works.
Sometimes you may have strange hangers that can only accommodate one carabiner, or maybe you're leading a roof where it makes more sense to clip the quick draw first, but those situations are pretty rare.
When you place a cam, you usually (depending on cam design) have three different options of where to clip it:
the racking carabiner
the sling
the wire loop
The wire loop is generally best for aiding, because it gives you the highest reach. (Note that some cams, such as the Metolius Master cam, don’t have a wire loop.)
Note that clipping the wire loop decreases the rated strength of the cam by about 1-2 kN, due to the sharper angle. But, it's just for your body weight and not for taking a fall, so in this case it's fine.
Let's look at these three clipping options in the photo below.
Left: Aid ladder clipped to the racking carabiner of the cam. Not ideal, you're losing about 8 inches of reach. Clip it higher if you can.
Center: Aid ladder clipped to the sling. Slight improvement, but let's see if we can get a little higher.
Right: Aid ladder clipped directly into the wire loop. This gives you the most reach, and is usually the best option.
While most modern cams are designed with wire loops that can handle your body weight and even maybe a bounce test without getting too abused, older ones (below) may not. Repeated aid climbing can damage the cable housing and potentially kink the cable a bit. Newer style cams with more robust thumb loops generally don't have this problem. Here's a photo of my beloved old orange Metolius that’s not in the best condition. =^(
