If you find yourself constantly commenting and uncommenting bits of code during development, this tip could come in handy.
Normally, you'd do something like this:
#define /* #undef */ _FOO
bool x = true; // = false;
Instead, use C style commenting to your advantage:
/**/ #define /*/ #undef /**/ _FOO
bool x = /**/ true /*/ false /**/;
This defines _FOO and sets x to true. Deleting or adding the second slash toggles the comments:
/** #define /*/ #undef /**/ _FOO
bool x = /** true /*/ false /**/;
This also works when you have large chunks of code you wish to "toggle" and a preprocessor define would be overkill. Keep in mind this makes code harder to read to someone not familiar with the technique, so let your team know how it works and use it sparingly!