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The easiest way to do what you're looking for is to call spinOnce in a loop. This is non-blocking, and allows you to do other operations in "parallel" with the callback processing. Note that this is not truly parallel, since this is a single-threaded model, but many applications are fine without truly multi-threaded operations.

See here for more discussion of various spinning methods.

Try something like this:

my_callback(){something};

int main(){
  sub = node.subscribe("topic", &my_callback);

  ros::Rate r(10); // 10 hz
  while (ros::ok())
  {
    do_callback_independent_processing();

    if (new_callback_data)
      do_callback_dependent_processing();

    ros::spinOnce();
    r.sleep();
  }
}

See here for more discussion of various spinning methods.

Try something like this:

my_callback(){something};

int main(){
  sub = node.subscribe("topic", &my_callback);

  ros::Rate r(10); // 10 hz
  while (ros::ok())
  {
    do_callback_independent_processing();

    if (new_callback_data)
      do_callback_dependent_processing();

    ros::spinOnce();
    r.sleep();
  }
}

Edit:
If you don't like the idea of calling spinOnce in a loop, then probably the next-best choice is to use ASyncSpinner. Trying to use multiple threads and multiple calls to ros::spin() is messy, and probably won't work. ros::spin explicitly states that it is designed for single-threaded nodes only.