How do I maintain a Brooks leather saddle?
Brooks makes a product called Proofide for saddle care.  For normal use, once or twice a year is likely enough.

To use, start with a dry saddle and wipe off any grit or dirt with a dry rag.  Then take a very small dab of Proofide and spread it on the top surface of the saddle.  You want to spread this out into a very fine layer all over the top (finished) side, don't use too much.  Then let it dry -- probably takes 4-12 hours, depending on how much Proofide you used.  Finally, polish the saddle to get rid of any excess Proofide (looks like a white waxy residue).

There's a Brooks maintenance kit that includes a tin of Proofide, a polishing cloth and a tension adjustment wrench.

Also, try to keep your saddle as dry as possible.  Riding in the rain is ok, but make sure you store your bike somewhere that the leather can dry out naturally.