I find boiling water works best... it's how I got my surplus boots to stop looking so 'out of the box' fresh
I'd also suggest an old tooth brush. It'll do less work than sandpaper would, but it's good for removing the sheen off of new leather and adding some light scratches.
Files also work well but you have to be careful. If you have too heavy a hand and are using a metal file, you can gouge big chunks out of the leather
If you boil it it'll become really hard and brittle, I wouldn't recommend that at all. Warm water, sure, but not boiling. Sandpaper or a light scratch pad work very well.
Many leather conditioners, like mink oil, will darken the leather just a little bit.
Products with hydrogen or benzoyl peroxide will bleach leather. Other types of bleach will not work. I think that Retrobrite might work. I have not tried Sea Glow, but I have heard that it could leave stains.
Chlorine will destroy leather, it will wither, shrink and dry out -- use only if you want the leather to be withered. Very dangerous.
I spent a good few years doing historical re-enactments, and I found that the best way to make leather look old was to give it a good going over with a fine wire brush (a suede brush will do) to take off the shine, then rub in plenty of dubbin. Not only does this darken it down, the dubbin waterproofs and softens the leather as well.