How do drugs used to treat heart failure modify preload and afterload?

Answer 1

Depends on the drugs...
Preload is the amount of cardiac muscle fiber tension (how much more can the muscle fibers stretch) that exixts at the end of diastole (b4 contraction)
Afterload is the force the ventricles must overcome to eject their blood volume
Class II Drugs (Beta blockers) block beta receptors which reduces the heart rate and the contractility of the muscle fibers.
Class IV Drugs (Calcium Channel Blockers) slows the calcium ions from entering the cells. This also reduces contractility.
By changing or reducing how the ions flow into the cells you increase/decrease the time for polarization(contacting)/repolarization(resting), which affects how/when the contraction occurs.

Answer 2

Most of them work to increase the contraction strength of the heart, rather than actually changing preload and afterload. They decrease the heart rate, increase the strength of contraction, and therefore increase the ejection fraction of the heart - decreasing afterload. :)