!
Finally, something I know something about. I've aged paper a bajillion times, because I'm a total dork when it comes to authenticity in my campaigns. I really dig actually handing a player something.
Luckily, most of my players are craftsmen of some sort (I work in print media, one is a carpenter, one a blacksmith, etc.).
As for paper, use the following:
Tea Bags - Using moist tea bags, drag the bags across the paper, getting it damp. It'll also lend the paper a yellow shade, and some brittleness, incresing more and more as you use additional "coats".
Edge Burning - This is probably best to do before you write, unless you can blow out the flames quickly. Simply light an edge aflame, and blow it out before all is consumed. It's best to use a match for this, and you can even drive the flame around by tilting and turning the paper. Remember, things always burn upward.
You can also make a burnt hole in the page by stabbing it with a lit match, and blowing out quickly.
Starch - Get some shirt startch, and dilute it with a good quantity of water (three-four cups). Soak the paper in this, and then let it air dry. Keep in mind, though, the paper WILL break if handled roughly.
And a note, be really careful when using the various liquids, as ink WILL run. It'll make it look more authentic, just make sure things don't get unreadable.
Have fun, most importantly.
