Not sure if this will be useful, but IF your starter is similar to mine in a 1990 3.3 litre van (also like Toyota, etc), you can take the solenoid apart and carefully file down the copper contacts and the copper on the plunger thing (it must have a better name than that...) all of which will be pitted and worn and that will help. If you want a more permanent fix, take the contacts out of the solenoid and silver solder over the surface, and then file them down nice and neat. They'll last a long time then, silver being harder than copper.
Dave