Downshifting tension Can you adjust your (Shimano) gear shifters to have less/more tension?  I'm frequently downshifting from high to low gear and would like to reduce the amount of finger energy required to downshift.
I'm pretty sure the amount of tension required to downshift is almost entirely determined by the derailleur's spring. Try down/upshifting without the derailleur attached (just holding the cable in one hand whilst shifting with the other), it will take almost no force to do so.

Cable condition can have a big impact on how much pressure it takes on the shifter to shift up/down. If this might be an issue for you try replacing the cable (either inner/outer or both depending on how good/bad your cables are).
I've recently replaced the cables of the front derailleur of my mtb which were in very bad condition even though they looked ok visually. It took so much force to shift (both up/down) that I feared I might break the shift levers. After replacing the cables it shifts like new.

Cleaning/lubing your derailleur might also help make it easier to shift up/down.

If your derailleur limits are not set properly this might cause the up/downshifts at the extremities (lowest ad highest gear) to be more difficult/take more pressure to shift due to the limit screws blocking/halting the movement of the derailleur.

As far as I know there is no such adjustment on shifters since the force required is not determined by the shifter but rather but the force required to move the derailleur (and also by the leverage ratio your shifter has (amount of movement of the lever compared to amount of movement of the cable caused by this lever movement) but this is -as far as I know- non adjustable.