Suggested Leather care from Dwayne.

Upkeep: Occasionally wipe down with a good quality leather conditioner, Sno Seal, Aussie Leather Conditioner by Fiebings, etc.  If the main ingredients are beeswax and oil then your good to go.  Avoid the cheap stuff as its mostly water and/or petroleum based.  Avoid cheap neatsfoot oil as its mostly petroleum and will  degrade the sheath with time, and over soften it.  Products like saddle soap are great as long as you look for quality.  Good places to look for leather care products: Your local leather craft supplier, boot and shoe shops, some boot and shoe repair shops will have a selection of the good stuff.  When applying said conditioners think moderation, a little dab will do ya, glopping it on will only over soften the leather and then you'll have a floppy sheath. 

If your sheath gets wet simply set it in a cool dry place (no knife in it) and let it dry SLOWLY.  The key here is letting the leather dry very slow so the oils don't get pulled out.  After the sheath is totally dry then you can apply a small amount of conditioner.

If you want a shiny finish use a good quality boot wax.  The soft waxes are better but not necessary, kiwi makes a totally serviceable wax.  Most all of my sheaths (unless special ordered with a different finish) are wax finished, so a good waxing will actually be beneficial.  You can choose a wax that matches your sheath color to help cover dings and scratches like a touch up paint.

If the belt loop gets floppy and soft simply run hot tap water over it till it soaks up a good amount, mold it to the desired shape and let it dry in the aforementioned cool dry place and voila! It will harden back nice and stiff.