Top spar bow up, bottom spar bow down.  That way they cancel out.

Take your spar and with the bowed side up, rub it against the back of a smooth rounded surface like the back of a varnished wooden chair or table. Use a back and forth motion with medium pressure. Your bowed spar will magically straighten itself. It's not a 100% guaranteed procedure but worth a try and pretty cool when it works. Otherwise, the above advice is worth following.  

I've done this, I've also just flexed by hand until straight, or over bent so it relaxes straight. The best way I've ever straightend wood is to soak it in hot water in the bath tube until its pliable then straighten it by hand and hang it vertially to dry.

   Do you fellow's glue your spar's down when they are slightly bowed?  I am in the process of building a Barnstormer wing and the spars are slightely bowed and I don't want to create a stress in the wing trying to get them into place..
  Sure could use some suggestions..  Thanks a lot,

From a purely woodworkers point of view, the theoretically best way is to split the bowed piece of wood down the centre, flip over one of those cut pieces so the bows are back to back and laminate them with glue back together so all the stress stays internal.

But hey, its hardly worth it when you have the opportunity to mirror that timber with a second separate spar although that keeps the stress external.