The more you work at the craft, the more you prepare fertile ground from which the good spontaneous songs come. Typically, you will work at crafting several songs over a season, and then one day out pops a beauty with little effort. Your hard work pays off by conditioning your songwriting muscles. Eventually, you won't have to work so hard. In my own particular case, I sweat over lyrics.
Music comes naturally.
Every once in a while, a good set of lyrics will pop out without me having to sweat over them, but this is the exception for me. A spouse is a good sounding board if he or she can give you positive and encouraging feedback. Find someone who is further along in the craft who can give you suggestions.