How do you preserve teak outdoor furniture?

Answer 1

teak oil from your local DIY store
rub it in with a cloth.

Answer 2

do not use linseed oil!!! it will turn your teak black!!

the absolute best oil to use is tung oil.
products sold as "teak oil" may contain tung oil. but will also contain cheaper oils, dyes, and other junk. to apply the oil:

clean the wood with a SOFT brush and some wood soap. a hard or stiff brush willl damage this soft grain of the wood. rinse it well. (you may have to repeat this a couple of times. if it is still really dark you may have to bleach it with a teak bleach sold at boat supply stores)
after it is clean let it dry.
once its dry use a brush to apply tung oil that has been diluted 50% with mineral spirits. let it sit for a few minutes and then wipe of excess with a rag.
after it is dry- sand LIGHTLY with 100 grit. then use a brush to apply more tung this time diluted around 80%.
repeat- wipe, dry,sand and then finally apply one more 80%.
wipe down after a few minutes with a rag moistened with mineral spirits and let dry.

it is much easier than its sounds and the results are fantastic.
i have worked with teak on boats for a long time.

Answer 3

Linseed oil works well. Just pour a little on a cloth then wipe it over the furniture. Linseed oil works really great on wood furniture also.