What product will make unfinished pressure-treated wood last longer?

When it comes to pressure treated, I’ve done it all—stains, paints, oils, transparent, solids—and not been happy with any of them. However, I’ve come up with the following regime that works very well: I pressure wash the deck clean, then apply a spray-on coat of Thompson’s WaterSeal. That’s it. While the water seal won’t protect the way a film forming finish would, it doesn’t need to be stripped every couple years either. What it does do is provide a barrier to dirt, which makes it easy to pressure wash the deck clean in a year or two when it starts to look grungy. After that washing, simply spray on another coat and you’ll be good to go for another couple of years.