Wood
To accomplish an "antique" look, sand wood surfaces, knick them with sharp objects like screws and nails, pound them with hammers and make nail holes in them. To further mimic older pieces, apply wood stain or paint in the same color as an older piece. Since the wood grain will be hidden by paint, add strategic black lines here and there and in nail holes to make the paint look worn. Crackle glaze can also be used over paint. Once dry, it looks like chipping paint.
Metal
The safest and easiest way to age metals is to use a combination of nature and natural solutions. You can leave many metal items outside and allow rain and harsh weather to cause them to rust. You can create spots of greenish-blue verdigris coating on new copper by soaking it in strong vinegar. You can also purchase special paints and solutions that react to give metals an artificial patina or rusty coating.
Pottery
You can age unglazed pottery by leaving it outside filled with mud and allowing nature to take its course. Within a short time, powdery white mildew will form on the outside and in a ring on the inside marking the mud level in the piece. Age glazed pottery by soaking it in dark liquid like coffee or tea, carefully burying it in mud until it stains, and scratching it up by rolling it in grainy dirt or sand.
Paper
You can give paper items an aged, yellowish or brown appearance by staining them with water diluted coffee, tea or craft dyes. Experiment between full immersion in the dark liquids and partial exposure using a sponge or rag. You can also crumble paper up and then re-flatten it to give it a distressed, aged look. You can also hold the edges over a candle to brown or burn them, making them appear older than they are.