I posted the following on Absolute Write in a discussion on extending the length of a story from novella to full novel length. All of this a just a retelling of some tips I have picked up along the way from various sources, and it describes some of the processes I went through in applying them to my own work.
AS AN EXPERIMENT:
Find a random page in the middle of your book, blindfolded if you have to.
Right at that point in your story, have something go wrong, even if things were
going wrong already. It can be anything in the context of your story, you are
the best judge, but make the train skip the tracks, even derailing. Are you
imagining something that could possibly go wrong with your story at this point?
Now, how does this effect things? What will the MC have to do to get things
back on track? What will the villain do to take advantage of this situation?
What will the MC's love interest think about this sudden change? Spend some time
resolving whatever went wrong, apply its effects to everything downstream where
it is applicable.
Now you have some idea of how to lengthen your story, you are not tacking
things onto the end, or pushing your plot past its logical conclusion, but you
are adding drama and adversity to the situation, now there is more that must be
resolved and a greater challenge. This means some significant re-writes that's
true, but if you MUST get the size of your stories up, it may just be that your
plots right now are too easily resolved and can use a boost of conflict.
Are you holding on those peaks of drama? Take a look at a scene where the
drama has built to a crux situation. An example would be the bad guy cutting off
the hand of the MC and announcing that he is his father, bad day huh? How does
that scene play out, does your MC fall immediately as he screams "NOOO!" and
then you cut to the next scene of his rescue? Or are you holding on that moment,
what is going through his head as his world crashes down around his ears and the
pain of a severed limb competes for his strength and will? He is slipping, but
holding on, he is crying, but solidly resolute and spitting in the villain's
Find those places in your story, there should be several, even if they are
not that intense..it could be where your MC's dog slips her leash and runs off
into traffic. Initially you might have your MC take off running, catch the dog
and cuddle it in relief...but you could do all that while your MC's is imagining
the dog being run over, what will she tell her mother? She loves that dog! Her
day potentially went from good to horrifying in an instant, HOLD ON THAT. Not
for long, not even a 1000 words, perhaps just a few hundred. Now find more
places like that in your book, tune the drama, don't allow things to instantly
resolve without repercussions.
Both of these solutions require downstream re-writing, the second option is
the least intrusive because it can all be interior to the MC...but either will
yield better results, in my opinion, than tacking on more material at the
beginning or end of your story, before or after the logical plot. And niether is
simply padding the story, they are adding to the drama.