I know that this is a subject that has been covered before, and by those much more talented than myself, but I'm relatively new to the forums, and as I'm currently working on some Historic/Steamy documents, I thought I'd share what I've learnt.
I've tried the tea and coffee methods, but for me, being of a more impatient temperament, I find using a blend of watercolours quicker, and you can control the results to a greater extent (Darker/Lighter, blotchy areas etc).
At the moment I'm using a burnt umber/ yellow ochre mix (Approx. 2 parts BU to 3 parts YO). This is applied to a fairly wet sheet of paper using a washing-up sponge. I do all this on a large melamine tray to minimise mess. Don't aim for a uniform colour or texture, any uneven patterns add to the effect.
A nice effect is to sprinkle salt on the damp paper, once you've applied the watercolour. This draws the pigment to the grains of salt, creating an interesting blotchy look (Rough pale circles surrounded by a halo of darker pigment). To my mind this is a nice nod to the effect that moisture has on old documents (That damp, slightly fungal thing they have going on!) This is definitely part of the process where less is more, be subtle!
Something else I've experimented with is ageing the paper first, and then printing onto it. Using a fairly good quality printer paper (I'm using HP 100g/m2. Well that's what it says on the packaging!) you can use any of the processes above, and with a bit of care, these pages are suitable for printing on. I'm using a Canon Pixma, I have no idea what would happen with better/worse printers/paper, so I hold no responsibility for any disasters! Of course, I hold full responsibility for any fantastic ground-breaking discoveries
If you carry out the described ageing process on blank paper, and allow to dry thoroughly, the result should be a nicely aged sheet, that's only slightly warped (I have found the papers tends to warp in large waves, rather than the small cockles that plague watercolour artists).
Making sure the paper is absolutely dry, brush off any loose pigment or grains of salt (I imagine these would be unhealthy for a printer). Then quickly run a hot iron over the paper (About 2/3 to 3/4 the full power). Also, set your iron to NO steam, this will cockle the paper.
The paper should now be good to go through a printer. Go wild in photoshop or word, producing Airship Docking permits and Ghosthunter advertisements
Here's a shot of the resulting paper (Highlights the texture better than the colour due to my scanner, though it's pretty close):
Here's a shot of some paper used in an ongoing project of mine: