Get something with a copper compound as the major ingredient. I got some at Loew's Home Center but cann't remember the name.

kemp: After reading on the SQF forum and other research, I painted mine with semi-transparent stain. It waterproofs, too. There are many shades. Check out HD or Lowe's.

kinda like the plastic idea

i just built some raised beds with untreated lumber, don't like the idea of treating the wood with a lot of chemicals....coating with platic seems like a good idea

I have many untreated beds going on 9 years old. I think plastic will only serve to trap moisture and facilitate decay. Plus plastic turns into a gazillion mini pieces when exposed to sunlight too long.

If you want, consider flipping the side contacting the ground each year.

Well....if I have to "replace" boards on the beds every 9 years or so I guess that works!!