A recent conversation regarding customization and rules abuse to obtain certain optimization benefits got me thinking in the need to balance skill point assignment. Optimization occurs when a player customizes a character in an extreme way that leads to game balance issues. In the particular case of D&D I believe this is due to the linear nature of skill rank and point assignment. There is no increased cost for improving a rank as long as you're level is high enough. So going from 3 to 4 costs the same as going from 8 to 9. This makes it very easy for a player to optimize certain skills and become ubber good at something.
If costs were cuadratic then it would cost considerably more to go from 8 to 9 than 3 to 4. Quite prohibitively so. Players would soon reach a point in which raising the rank of a skill does not provide the benefit enough to justify the cost. It is much better to improve in other fields that would be cheaper and give a better point to benefit ratio.
As I solution I've been working on an XP tax system that accounts for the character's constant training to keep skills in shape. The more you're skilled at something the more you'll have to pay to keep up that skill. So overly skilled characters would pay a heavy XP tax. This tax is literally deducting from XP assignment. If you earn 2000 XP in an adventure and your tax is 10% you only get 1800 XP.
This makes players think twice about optimizing a value. As a high skill doesn't only cost a lot to get it keeps costing on and on and on. The higher XP tax derived from that skill affects all gameplay. So a high skill kept to be used in combat for some specific advantage costs the player a lot of XP even if it wasn't used at all during a particular adventure.
Obviously this does not prevent optimization, but is sure promotes its disuse. Players that are willing to pay the high price for an overly optimized character are free to optimize, but their advancement will be hindered in comparison to others.