What can I do to prevent a brick patio from getting
Mold or mildew again
After powerwashing the patio.

I am assuming you are talking about house siding, right? If not, please write back. If you used a mildewcide it should kill not just clean most of the mildew. A periodic spray of a 1:3 solution of bleach/mildewcide and water will prevent regrowth. If it is on a roof you can install a copper or zinc strip at the peak. The rain reacts with the metal and prevents mold and mildew.

Prevent mold and mildew on what kind of surface, after powerwashing ? It makes a difference to know; it could be a porous surface which would have to be treated differently then non porous.

I am assuming you are talking about house siding, right? If not, please write back. If you used a mildewcide it should kill not just clean most of the mildew. A periodic spray of a 1:3 solution of bleach/mildewcide and water will prevent regrowth. If it is on a roof you can install a copper or zinc strip at the peak. The rain reacts with the metal and prevents mold and mildew.
What is the min. width of the copper strip for it to work, thanks

The copper is slid under the cap row of shingles and then leaves a reveal of about 4 inches. I do not honestly know if there is an actual standard. Maybe a roofer could help us out more.