The black art of suspension set up.....
Ideally the sags should be set first, thats the amount o the suspenion settles under the bikes own weight 23 - 28mm is a good starting point, then add the rider and look for 5 - 7mm more. Compression and rebound damping will be dependant on how the bike feels following that, you dont want it slow to respond in either direction but likewise you dont want it to 'blow through' the stoke too easily.
Suspension set up is all about feel and feedback, you need to know what your tyres are doing and the level of grip you have available.
If all of the above is in the too hard basket go along to a good bike shop or a know suspension tech and for a small fee they would set it up for you. Nothing could be worse than riding a bike that is poorly set up. Personally I would spend money on improving suspension first over any other 'farkle' or tuning mod.