Perhaps everybody else already knows this and as per usual I am late to the party but I noticed something very significant about skill ups.
You must(!) keep your weapon fully repaired or close to fully repaired to receive optimal skill up points from killing mobs. There is a significant drop off as the weapon wears out even if you are killing the same mobs as before.
The repair level of the weapon certainly has a strong impact but I believe that the repair level of the rest of your gear is also a contributer.
Also, using a weapon level significantly below your Rank will also diminish the skill ups that you get. Not the frequency, but the size of the number.
SO for all you people out there complaining about the drop off in skill points as you level up. Make sure you are using a well repaired weapon and make sure that it is close to your level.
On my Rank 12 Thaum, I was getting average skill ups of around 60 per battle with a virtually destroyed weapon, then I repaired my weapon and this increased to around 150. I then upgraded my weapon from the level 1 weapon that I was using before, and started getting around 250 per kill. (same kills, grinding in the same spot)
On my Rank 17 Conj, I was getting around 100 per kill using a trashed level 6 weapon. Repaired it and increased to around 200 per kill, upgraded to a higher level weapon and started getting around 350 per kill.