If it's a short story -
- Stick to the point. A short story doesn't have space to go on tangents. What is the main story you are trying to tell? If there's more to tell, perhaps that extra character or subplot needs its own story. Or perhaps you're not writing a short story at all, but something that should be developed into a longer format.
- Make every word count. Tighten your sentence structure. Make images work double duty - can the action be both progressing the story AND indicating character development? Can setting show the environment AND emotional state?
- Build toward the ending.
If it's a novel -
- Make sure every scene builds tension in some way. That doesn't necessarily mean physical threat (although it can do) but also emotional tension.
- Make sure something happens in every scene. This may seem obvious but writers often get bogged down because their characters aren't actually doing anything. Even if it's a scene where they're just talking, building relationship or revealing backstory, make sure they are also doing something specific and interesting. Have scenes serve double-duty where possible.
- Be original with the settings and activities. Give the characters specific things they like to do and show them doing those activities. Send them out to unusual places. It doesn't have to be outlandish, but it should be interesting.
- Have a hook at the beginning and end of scenes. Encourage people to keep reading.
- Mix it up! Lay out the scenes and consider how they flow. If there's a section that looks too low energy, swap the order around to give it a boost.
Whatever you do, don't add words just for the sake of extending the story. If you're writing for a longer market then add action, subplots, and character development - not just padding. And if the story finishes before you thought it would, but it's a good, tight story - then celebrate. You can always find another market that suits that length.