If any of the decking needs replacing, do so. Other things to do to waterproof it for good:
Snow & Ice Shield - A rubber-like membrane that goes right on the decking before you shingle. Assuming you have the minimum pitch for shingling, you can do this. Roofin supply houses such as Bass Supply carry this membrane. It is sold under other makes and mfgrs. in the home centers but just as good. Flashing - Flash where the porch roof meets the main house. Make sure that if you're using aluminum flashing you're using aluminum nails. Same goes for copper, etc., etc.. Quality Shingles - Again, assuming you have the proper pitch and are shingling, spend a few extra $$$ for a good shingle. Drip edge - Install proper drip edge. The drip edge on the rakes goes over the membrane and the drip edge on the eaves goes under it. Gutters & Downspouts - I suggest aluminum, contiguous gutters; not vinyl. Make sure when the water leaves the gutters it is a place to go. Downspouts should extend 3' or more, in optimum conditions, away from the house. You can opt for French draining or running the pipe further away from the house. Ventilation - You didn't say if the porch is enclosed & heated. If it is, optimal performance would suggest soffit vents and peak vents (one on each end assuming the porch is free-standing, so to speak.) This helps keep the underside of the sheathing cool in summer.
When I re-did the roof on our addition, I used Snow & Ice Shield on the whole roof because it faces North (little / no sun in Winter) and because it's just above the minimum pitch for shingles. I used 3-1/2" drip edge and heavily flashed around a skylite and where the roof meets the house. Finally, I had a contractor install seamless gutters.
I can't think of much else. My best to ya and hope this helps.