Well, a turbocharger basically relies on the expansion and energy of the exhaust flow in order to operate, so improvements in this respects will serve to improve the spool characteristics; so in laymen's terms, keeping the exhaust flow reasonably hot, and by improving exhaust velocity (think kinetic energy of the exhaust flow) will improve spool. So, typical things to help spool include increasing the C/R (more air compressed, hence greater rate/volume of expansion on ignition), headwork (increase the volume and speed of air flow), uprated camshafts (greater volume, and combined with some scavenging, greater velocity), advancing the timing (cylinder mixture is more fully burned, again creating a greater rate/volume of expansion), ceramic coating the exhaust housing/manifold (keeps the exhaust hot, maintaining velocity), selecting the right wastegate and boost controller and so on