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Already good answers, but I can provide a slightly different perspective here: always use braces if there is a risk of getting into a pitfall. 

Examples (from C#, but the language is less relevant).

- omit if the inner instruction fits on a single line and an extra blank line is inserted after:

      if (some_condition_here)
          return;

      var x = call(param1, param2);

- include if the inner instruction does not fit on a single line:

      if (some_condition)
      {
          var someResult = DoSomeSpecialProcessingWithLotsOfParams(
              param1, param2, param3, param4);
      }

Of course, it is easier to just always put them, but I see no point in having them for simple cases such as my first example.