Best improvements to do to the DPLL SAT algorithm
The most improvement with the least effort will come from adding rapid random restarts to your solver.  DPLL is known to exhibit heavy-tailed behavior, producing both short and long search times for instances depending on initial search conditions.  Random restarts allow the solver to bail out of exponentially long searches of unfruitful assignments and give the solver more chances to find one of those short searches sooner.

Implementing two-watched-literals will produce the next big improvement because it eliminates a major backtracking cost.  Backtracking is done exponentially often in the worst case, so any improvements there offer unequivocal improvements in search speed.  Implementing two-watched-literals will require a significant overhaul of your solver, but it will teach you a lot about how real-world SAT solvers work.