Imitating the minor scuffs and abrasions that leather experiences over time is an important part of making it look worn, and hence old. Locate the parts of the leather most likely to be on the receiving end of such wear; edges and prominently convex areas (such as the elbows on a leather jacket) are generally good starting points. Scratch up these areas with very fine sandpaper using light pressure. Be careful not to push too hard or use a coarse sandpaper, as that will look less like wear over time and more like damage from an individual event. Scratch up the entirety of the leather to a much lesser degree. Sand at a variety of angles to minimize the appearance of obvious sanding lines.
Pull firmly on the leather to stretch it in the directions and locations that it would undergo such stress in normal wear. Most leather, as long as it does not have a firm backing of some sort, will stretch to a degree. You may need to wet the leather to get it to stretch significantly, however.
Leather darkens and conditions over time through exposure to skin. If the leather in question comes in contact with skin and is not already black or close to it, rub some neat's-foot oil into it. This will both darken the leather and soften it, giving it a more worn feel.
Many items get stained over time through accidental contact with liquids. Think about the way your leather item would likely be used and with which materials it would probably come into contact. Rub or spatter some of these materials onto the leather and leave them there for some time to create stains. Be sparing with this, however, unless you want to make the item look truly messy.