What's the best way to clean outdoor wicker furniture?

If you think it is natural wicker then you can't use water because natural wicker will suck the water and then rot. If you are 100% sure that you have synthetic wicker furniture then you can use water with no problems at all. First take your cushions away. Then hose down the furniture.

If you have a lot of dust accumulated use a soft brush to brush out the dirt.

You can use non-abrasive cleaning products such as soap, Windex etc. Don't use anything close to bleach or ammonia. The chances are they will ruin your furniture.

Also avoid pressure washing. It will ruin your furniture. Use lots of water.