The best way to fine tune race hydration and nutrition plan is practice, practice, practice. One should not try any unfamiliar sports drinks or energy gels during the BIG day! Needless to say, the same goes for running gear, shoes and socks.
Furthermore, find out from the race organizers what sports drink they will be serving, and get used to it in other less important races or during long runs. Same goes for energy gels.
It also makes sense to practice drinking while running at a faster pace, and to in advance test how to best carry the energy gels when running at race pace.
My own very simple marathon hydration and nutrition plan:
1. Drink at every service station (one or two cups)
2. Make sure to drink sports drinks, not only water
3. First energy gel (25g pack) taken after 15 km
4. Then one energy gel every five kilometers until 40 km
5. Energy gel in-take just before service stations, immediately followed by drink