Possibilities may flow out, but the devil is in the cutting. You may have a large crop of nice rhymes and ideas that make decent couplets, but trimming them down, even if painful, can make the whole adhere to a good structure rather than just a collection.

A good structure, though depends on viewpoint; a rollick from bar to bar ends up in the jug, while a maiden's lament may never end in time although the song can end by showing the timelessness of it. Some songs have a surprise ending, or a sort of ineluctable conclusion where the end ("I'm the last of the Irish Rover!" leaves little to be said), while others refuse to end decently, of which the archetype is 99 bottles of beer.

The point is that the raw material of ideas will bubble around and evolve hopefully into a clean structure.

Two parts of good songwriting sometimes not noticed are the use of implicit images -- the things only suggested sketchily which provide a full image in the listener when well done) and on a different note the interesting adhesions and tensions that can come from internal rhymes, using syllables within the line, not because the end syllables gotta rhyme, but because the internal resonances make the body of the song sing more vibrantly or adhere differently.

An example of these from the "Irish Rover" is the passing reference to 48 bales of old nanny goat tails, or the "great load of bricks for the new City Hall in New York". The latter conjures up in one little line a whole image of the import of bricks from Ireland to New York and the situation of Irish immigrants using them to build Tammany's City Hall -- all by implication. And the internal rhyming in "bales of nanny goat tails" makes it flippant and funny.

These are some tricks I use and if they are useful to you, welcome to 'em, and if they are