I have noted on many sites that the vast majority of players either misunderstand or never even thought about what a "Nindo" is--beyond something Naruto shouts and RP sites require you to come up with. While a Nindo certainly doesn't affect a character's combat abilities I would argue that it is every bit as important to RPing said character as the personality or appearance!
Why, whats the big deal? The "big deal" is that the Nindo is the foundation upon which the ninja's actions rest and represents the deeply held belief that dictates how the ninja responds to the ninja world around him/her. Many people simply put "something cool" or a favorite quote and just leave it at that, but such a Nindo says nothing about the ninja or the motivation that drives them in their ninja life.
In defense of the masses the Naruto manga/anime only really delves into two Nindos; Naruto's and Kakashi's (though there are hints at others). A casual fan that simply enjoys watching/reading Naruto has little reason to see this as significant, but someone who questions and looks more deeply into the series will quickly start to see many unspoken implications/connections not readily visible to the average viewer/reader.
Consider Naruto's Nindo: "I will not go back on my word." At first blush this seems to simply be a coolly defiant catch-phrase, however if you remember that Naruto was always dismissed and looked down on the sentiment becomes much more clear. The insistence to not go back on his words fuels Naruto in his drive to be Hokage and forces others to recognize him by demonstrating that when he says something people need to take him seriously. It is more than simply a catchy quote; it is the foundation for every action he takes as a ninja.
Kakashi's Nindo is somewhat different in that it is not what it started out as but changed due to the events of Obito/Rin's deaths changing his perception of what it meant to be a ninja as well as his personal priorities as a ninja.
How do you make a good Nindo then? First you need to think about not only why the character became a ninja/what they want to accomplish but also about how their past may have influenced their purpose as a ninja. Second you need to consider how your character thinks they personally fit into the ninja world and where their priorities as a ninja rest (this part sort of "modifies" the first part). It is important to remember that a Nindo can be as complicated or simple as needed, but should represent a single core thought/belief upon which all the reasons and experiences which made it rest. If it is simpler though you might consider a Nindo a simple mantra to refocus the ninja on their core concept (often bolstering determination or courage).
A Character's history creates their personality and their personality dictates how they will act/react in the RP, the Nindo is something akin to the lowest common denominator when the personality and history is distilled down into a single concept (a sort of "one sentence summery" of their past and personality).
Anyway, just my thoughts on the matter.... Ignore me if you like.