As the Player's Handbook explains, this ability is not literally just the knack of hiding in areas of shadow while not moving. It also covers hiding in bushes, shrubs, behind drapes, in alcoves—anywhere where there is a chance of skulking about immobile in some location which offers cover and the hope of staying unseen. Considering appropriate equipment needs to be considered in this light.
Camouflage Clothing[]
There may be exceptional variants on this—for example, if a thief knows that he needs to hide in shadows amongst the crimson curtains of a lady's boudoir he might use a crimson set of clothing. Apart from such singular special cases, there are two general types of camouflage clothing the thief will find useful in various situations.
Darksuit: This comprises black or very deep blue clothes, usually fairly voluminous robes or what might be termed a "utility suit." Such clothing will usually be lightweight so it can be carried about easily, and worn under normal clothing if needs be. It will always include significant facial covering. If such a suit is worn, it will add +5% to a hide in shadows chance in any area which is shadowy, has a light level equal to dusk or early dawn, or equivalent.
Woodland Suit: This has a similar basic design to a darksuit but is made of light clothing dyed in irregular patterns of greens and browns. It adds +5% to the hide in shadows chance when the thief attempts to conceal himself in any suitable outdoors setting such as woodland, a field, or the garden of a town house.
Charcoal[]
Even wearing a darksuit, the glint of moonlight on a pallid white face can give a thief away. Blacking up the face (and neck) with charcoal adds a further 2% to the hide in shadows chance for concealment in shadows, dim light, etc. Burnt cork and soot are alternatives. The thief should not forget to blacken the backs of his hands either!
Plant Dyes[]
Analogous to the use of charcoal in dim lighting, a thief can use plant dyes (crushed leaves, berries, certain oily barks, etc.) to disguise his exposed face and hands and add a further +2% to his hide in shadows chance in suitable outdoor settings. Such plant materials are usually rather sticky and smelly, however!
Weaponblack[]
One common problem for the thief attempting to hide in shadows is the glint of a steel weapon in moonlight, torchlight, and the like. The way to avoid this is to use weaponblack, a thick, oil-based emulsion. Weaponblack can be coated on to any metal surface and renders it almost completely matte black and reflection-free. If the DM wishes, use of weaponblack can give the thief a +5% bonus to hide in shadows. A superior rule, though, is that use of this substance saves the thief having to make a second hide in shadows roll if he draws a weapon while hiding, or saves him from any penalty modifier if he has a weapon already drawn when he attempts to hide in shadows initially.
Once the weapon has actually struck a blow, the weaponblack will be wiped off. Because it is oil-based, it is also highly flammable. If ignited it will burn brightly, turning the weapon into the equivalent of a flametongue (although it does not count as a magical weapon) for 1d4+1 rounds. if the thief is not protected from fire (e.g., with a ring of fire resistance) he suffers 1d4 points of damage per round while holding the weapon.