Personally, I think if you intend to empower Sneak Attack you should create a cost that is as thematic as prohibitive. The first idea that comes to mind is multi-classing. It cuts back on the levels that grant Sneak Attack while also allowing for use of (at times limited) mechanics of other classes to empower the Sneak Attack.
A feat requiring loss of Psionic Focus in order to increase the die of the sneak attack means it cannot be used with other psionic feats that require loss of focus (at least unless you use up even more feats on psicrystals).
A feat requiring a use of Turning / Rebuking to empower the die of a sneak attack might also work. (cleric / rogue, maybe even paladin / rogue - although I would prefer decreasing smite evils for such a combination, as the thought of stacking a smite with an improved sneak attack seems odd and possibly overly powerful - and it discourages the class combination [which should be discouraged, perhaps] to require the use of a smite for improved sneak attack).
A feat that only increases the sneak attack hit die against one's Favored Enemy (rogue / ranger hybrid) could also work, as it limits it severely - especially if the feat requires choosing a favored enemy to use it against (from those already taken as such), but this latter might make the feat a bit weak.
A feat requiring a use of bardic music to increase the hit die of the sneak attack would be odd, but it could work. (bard / rogue)
Hmm, what about a feat allowing a wizard rogue or sorcerer rogue to lose a slot to covert that many dice of sneak attack into a higher level (ie: losing a 4th level slot to covert 4 dice of sneak attack)? Slots are as much a limited resource as anything else, although I would limit this to arcane slots - just to prevent turning / slot combinations that might otherwise stack for overly potent sneak attacks.
The Barbarian could have a feat allowing use of a rage attempt to - instead of rage - increase the dice of a sneak attack. Sounds odd, but perhaps it could be used somehow.
Druids might perhaps use a Wild Shape attempt to make the sneak attack, sort of in the sense of partial wild shaping (as described in UA, where only a body part becomes altered). So it would be described as the arm suddenly lengthening so as to allow an unexpected attack a foot closer / deeper than expected, or perhaps as claws forming that deal terrible damage, or so forth. It might be workable.
For the pure Rogue, perhaps a feat allowing the use of a feint (Bluff check) for increased sneak attack potency could be used. If it succeeds then the dice are increased. If it fails they are decreased - thus discouraging casual use at every single sneak attack attempt.
Stunning fist comes to mind for the monk, perhaps a feat allowing the monk to set up a stunning first attack but instead deal enhanced sneak attack damage if it succeeds.
That leaves the Fighter lacking a feat for increasing sneak attack dice. I'm not quite certain what to do with them. Perhaps something akin to Power Attack or Combat Expertise? Allowing them to decrease either their BAB on the attack or their AC for the round of the attack, at a -1 (-2?) per sneak attack dice enhanced? It seems a bit odd, but perhaps it could work.
In any case, these are my ideas on the subject of increasing sneak attack dice via feat use (ie: requiring multi-classing or having a notable penalty for failure [see pure rogue example] ).
Necrrromancy 
