1) Have the drivers rebuilt. After 20 years, the suspension of the drivers can often have stiffened considerably and the copper or aluminum of the voice coil may have corroded and lost a significant amount of conductivity it once had.
Find a reputable shop like Bill Legall at Millersound who can do the work, at a surprisingly low cost. Drivers are where the rubber meets the road, like the tires on an automobile. Though most overlook them, the results can often be quite surprising
2) Crossover resistors. Even the best resistors sound pretty bad, so spend a few (literally, only a few) dollars here buying the best.
There are a lot of opinions here, including those held by supposed speaker gurus. In my experience, silicone coated wirewound resistors are the least bad sounding of the lot.
Even better for those who can get away with it, consider removing the resistors altogether. This impacts the crossover parameters, however
3) Crossover capacitors. More detail, clarity, and air, though many "better" capacitors sometimes leach musicality and can impart a bright or even bleached out sounding presentation
4) Crossover inductors. Better copper smooths out the presentation, while larger AWG yields lower DCR (DC resistance) can substantially improve dynamics
However, one should be VERY careful in making sure you don't stray much from the DC resistance of the inductors you are currently using, as simply switching from copper to silver or going to a larger AWG will. Throwing off these values shifts the parameters of the crossover, and unless one is OK with reengineering the crossover values, I'd advise staying away from this type of change
5) Binding posts. Edison-Price and Cardas unplated copper are the best