I second the Cetol.
But I've found after you sand the teak smooth, you can put 3 coats of Cetol on one right after the other with no sanding between coats, with about an hour or less between coats, dry over night, then sand with a scuffy pad and put 3 more coats on and your done.
The key to longevity is sanding the teak smooth before you start putting coats of Cetol on.
Here is my recipe for home made 2 part teak cleaner:
Just mix 1 cup TSP (trisodium phosphate, available at most hardware stores in white powder form) and 1-3 cups ammonia with 3-4 gallons of fresh water. That is your part 1.
Wet the teak and scrub with the part 1. Keep teak and surrounding areas wet but do NOT wash off part 1.
Mix 2-3 cups muratic acid with 3-4 gallons fresh water. That is your part 2.
Spread the part 2 on to the teak and lightly scrub. The teak will go blond.
Be sure to get the part 2 on all the teak that has been scrubbed with the part 1.
After you have lightly scrubbed down all the teak with the part 2 rinse everything off very, very well.
Since you mix this yourself you can delute it down to any strength you wish and it costs
about one tenth or less of the permixed kits.
It should go without saying you should use hand, eye and body protection when using this
or any other harsh cleaner. And it should not be used to clean the teak on a regular basis.