It is easy for setuid programs to give the user access that isn't intended---in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:
Don't have setuid programs with privileged user IDs such as root unless it is absolutely necessary. If the resource is specific to your particular program, it's better to define a new, nonprivileged user ID or group ID just to manage that resource.
Be cautious about using the system and exec functions in combination with changing the effective user ID. Don't let users of your program execute arbitrary programs under a changed user ID. Executing a shell is especially bad news. Less obviously, the execlp and execvp functions are a potential risk (since the program they execute depends on the user's PATH environment variable).
If you must exec another program under a changed ID, specify an absolute file name (see File Name Resolution) for the executable, and make sure that the protections on that executable and all containing directories are such that ordinary users cannot replace it with some other program.
Only use the user ID controlling the resource in the part of the program that actually uses that resource. When you're finished with it, restore the effective user ID back to the actual user's user ID. See Enable/Disable Setuid.
If the setuid part of your program needs to access other files besides the controlled resource, it should verify that the real user would ordinarily have permission to access those files. You can use the access function (see Access Permission) to check this; it uses the real user and group IDs, rather than the effective IDs.