One thing that you really need to do: coat all your cuts with some
form of Wood Preservative. They one I used on my fence, and plan to
use on the deck is Copper Green, which contains Copper Naphthenate.
Here is one link over at our Amazon
affiliate so you know what you are looking for:
Green Products 210-294 Wood Preservative, Copper/Brown
Although they make a brown product which I used, the color fades anyway,
so I wouldn't worry about the color. But it treats the cuts to prevent
rot. The middle of most PT wood is not treated, so when you cut, you
are exposing the wood to the elements.
There are other wood preservatives. I'm not aware of any definitive comparison of these products, so all I can say is I have had good experiences with this one.