"...Speedlights are designed so that the capacitors remain fully charged (to over 300v) when the unit is on. So working your way up from low power can actually be a problem, as you are leaving the cap in a near fully charged state most of the time. Which can cause a thermal runaway.
The process is just as easy, just a little different:
1. Turn on the speedlight and set it to full power.
2. As soon as the capacitor charges up (ready light glows) fire the flash.
3. Repeat the process.
4. Alas, speedlights do not dissipate heat very well when popped repeatedly at full power. And the last thing you want here is heat build-up. You may wish to turn the flash off and let it rest and cool every twenty pops or so.
5. This technique will build up the aluminum oxide layer on the cap (assuming it was not too far gone) and breathe new life into it..."
Update: Nikon no longer offers the cable upgrade to the SD-8 battery pack. I must have depleted their supply of replacement cables.
A final note: You will increase a speedlight's life expectancy by using it at regular intervals. Try to "pop" the unit a few times every monthly to keep the capacitors fresh.