They must be cleaned well.
remove the cables, scrape or brush all bad spots until clean, then wash the
terminals and cables with baking soda mixed with a little water to make a
paste.
then rinse with plain water and reconnect all cables(do not overtighten side
terminal types as they have a tendancy to pull loose from the battery.
This is a normal old fashioned way of cleaning and works well.
old john

Conductivity is not achieved through the threaded screws. It is the cable
end and the connection point on the battery that must be clean and shiny.
You can buy a side post brush or just scrape them with your pocket knife,
like I do. If, after cleaning, you still have the same problem, corrosion
may have gotten under the cable insulation, in which case, a new cable is in
order.
H

(Following by Lee Swanger from 62-65-dieselpage-com)
Use two nuts and a 1" or 1-1/4" long bolt; strip the insulation off the flat
connector at the end of the battery cable; use two wrenches:

1. Thread first nut all the way onto the bolt, not tight
2. Slip the flat connector onto the bolt
3. Thread second nut all the way onto the bolt, not tight (flat connector
now between the two nuts)
4. Thread the bolt "assembly" into the battery side terminal, slightly tight
but not too much or you'll strip the new threads (use wrench on bolt's head)
5. Back off the second nut until it is snug against the battery side
terminal, slightly tight (use one wrench on second nut and another wrench on
bolt head to keep the bolt from turning)
6. Back off the first nut until it and the flat connector is tight against
the second nut (this you can tighten more; again, use one wrench on first
nut and other wrench on bolt head)

Don't use grease or non-silicone lubricants on the threads because you want
maximum metal-to-metal contact for conduction. After you have assembled the
battery cable to the battery, you can coat the connector/cable with the red
spray-on battery protector available at most auto stores.

*******

If the bolt is loose in the battery terminal to begin with (stripped threads
in the battery), you could use a 7/16" bolt to replace the OEM 3/8" bolt.
Make sure your
7/16" bolt has the same TPI (threads per inch) so as not to cross-thread the
battery side post. Chase a new thread into the battery side terminal with
the 7/16" bolt.

Good luck. Franko...

Throw away the lead washer in the terminal.
Replace it with a harder material such as brass, a brass reducer cut to length works well.
Replace the bolt with on of the proper length, or cut one to length.

No grease or Oil, these will actually cause corrosion.
Petroleum jelly or the spray on protectant are a better option.

My rate is $110/hr, email me for payment options.
;D

Cheers