To really preserve your strings and keep a strong, constant tone, do the following:  Spray a very small amount of "Finger-ease" on a soft cloth and, avoiding the soundboard area completely, pinch each string and slide up and down cleaning with the applied area.  Afterwards, wipe down the strings with a dry part of the cloth to remove any residue.  It's also recommended that you do this to brand new strings before putting them on the instrument, removing any chemical treatment they have had from the factory.  This spray works on all types of strings for all instruments.

Every time I change my strings (about every 6 months or so) I apply some "Hill" brand peg compound that you can buy at any violin shop. I apply a little around each peg at the two "rings" where it is visible the peg contacts the pegbox, then I rub a little "Ivory" brand soap (99% pure soap) onto the applied peg compound which will then mix together for a smooth yet stable turning peg.

You may notice black marks on the face of your instrument under the treble strings where the neck meets the face, and elsewhere.  This can be removed by using 000 steel wool very, very gently and lightly rubbed along the grain, but please, BEFORE YOU EVER TOUCH THE FACE OF THE OUD, CONSULT WITH AN OUD MAKER/LUTHIER FIRST - I personally don't mind the marks and leave them alone on my instruments.  

So you know, you should also never put any oils or finishes on the oud face.  It will stain permanently, and possibly negatively affect the oud's sound.

Never leave your oud in the car, in the sun, by a heater, anywhere it might be exposed to too much heat, cold, sunlight, etc.  Extremes are bad for your instruments.  

Keep an eye on the humidity.  Too much or too little is not healthy for the woods on your oud.  Try to keep it around 40-60% or so. 

Always have a hard case for your oud.  You would make your child wear a seatbelt wouldn't you?  Protect your oud(s) too, they deserve it.  

It doesn't hurt every few years to have your oud examined by its maker, or by another experienced luthier/oud maker for a 'check-up.'  This is especially true for older instruments.