The relationship between HP and torque is simple:
HP = (Torque * RPM) / 5250
Based on the formula, as the engine's RPMs are increased, the HP generated increases. The limits of HP are set by friction, both within the engine and external (wind, rolling resistance, etc).
You can also increase your horsepower by increasing the torque applied to a/the driven hub. Two routine ways to do this: 1 - alter the drivetrain and/or 2 - alter the internals of the engine.
Torque is (force x distance from axis of rotation). As someone noted above, if you push on the free end of a 2' long wrench with 10 lbs. of force, you are applying a 20 ft-lb moment to the axis of rotation (the bolt/nut/whatever). Considering this, it is possible to increase your bike's "seat of the pants" torque by increasing the size of the rear sprocket. Doing so increases the distance thru which the force (the drive chain pulling on the sprocket) is applied, thereby increasing torque generated about the hub of the rear wheel.
One way to increase the amount of real torque that the engine produces (measured at the crank, not at the rear wheel) is by altering the stroke of the piston. To keep it short, the longer the con rod the greater the torque generated. Consider a stroker Harley; the con rod is repositioned further from the crank shaft's axis of rotation, thereby increasing the "distance" part of the T=f*d, thereby increasing torque and, consequently, HP. It's not that easy in real life; some other things have to change as well, but that's the bottom line.
If I missed something or misstated something, somebody please let me know -