As an aircraft mechanic, when in the field without resources, I've found that most flare fittings *except oxygen lines fire hazard* I can rub my clean fingers on the crease between my nose and cheek. Or, rub my forehead and wipe my fingers on the flare surfaces. The film helps the metals slide to seat when tightening. The threads may need a little too. Because the threads are supposed to be dry, sometimes the lines just won't line up good enough to start threading without a little help.