1.) Good voltage
2.) Good amperage
3.) A solid ground return to the battery.
Short form = "4.) All of the above" A healthy battery and good cabling (incuding the ground /negative lead from the engine block to the battery) will make any starter crank better.. This includes clean electrical path from the battery clamps to both the battery and the cable itself. Of course, if your battery has a dead cell or two; solid gold 4/0 welding cables from the battery aren't going to feed more than 9 volts or so to a brand new starter, so.......
Having said all that, if you're using an el cheapo rebuilt starter that barely can handle cranking over a never touched stocker 289; or if it was a really good starter when you bought it (20 years ago); well it just might be time to replace it. When you do; look at one of the "mini-torque"-type starters from a late model 5.0 Mustang. With their permanent magnets and gear-reduction systems; they could crank over practically anything without sucking your battery dry in 20 seconds. They're usually a lot smaller and lighter than a 60's-70's starter, too. This can be a good thing when trying to mount it up behind your longtubes; especially at midnight, in the Kroger Supermarket parking lot, with your 8-month-old baby screaming and crying in the car seat.
Do you really have to ask how I know?