How can I make a story bigger?

Put it away for a while. Long enough to not have it in mind to the level of daily obsession any more.

Then, dig it out and look through it. You may find that magically somebody has replaced all of your rich, description, amazing detailed characterisation, fascinating dialogue and lushly abundant prose with stuff that is a bit thin, doesn't always make sense and fails to really put your point across.

If this happens edit from there.

If it doesn't the story is, I'm afraid, already at optimum length.

EDIT: This seems to have become a really popular question and, as someone who is never short of words, I have been thinking the matter over for the last few days. If you're happy with the original answer above this won't add much more and should therefore be regarded as an optional read for those who really really care about the issue.

The first thing to bear in mind is that you can make a short story longer and still have it captivate an audience... but only as long as the shorter story was captivating to begin with. The longer story will only be as captivating (or maybe a little more) as the original was. The reason for this is simple. If you have it in you to write one fascinating story the chances that you have only that one story and no more are pretty slim. Stories come from experiences, specifically your unique experience of the world. However long it takes to write a story is nothing compared to the vast amount of life you live that is not about writing that story. So you just need to mine out some more story stuff to put into the story.

There are a couple of techniques that aid sensible expansion but none of them are a magic bullet. What these amount to is you allowing yourself room to do other things. I shall explain as I go:

a. Write in the third person - a first person story allows for only a single strand. With a third person story you can have multiple lead characters and essentially interweave three single-point perspective novellas into one novel.

CAVEAT: As soon as you do that you need to juggle timelines, consider who is doing what and when and make sure that it all hangs together.

b. Write the villain's storyline - You may not end up keeping it in but really getting into the villain's head helps you to notice gaps in the hero's story (and vice versa). Also there is a tendency for stories to employ villainous action in the manner of a story requirement for complications to arise. The villain has a plan, a life, a motivation and will do things whether the hero is there or not. If you know what those things are it may give you ideas for relevant and intriguing incidents in the other story tracks.

c. Think like the ultimate motivational pedant. The number of novels I have read where the author has done a serviceable job but left gaps in the hero's motivation for certain actions is very, very high. A hero should have impeccable behaviour and, where they don't, the explanations for why not bear serious examination. Regard your characters through the eyes of some naive but emotional moral observer and see if anything your protagonists do could be viewed as a bit wide of the mark. It's those moments that may require expansion to explain motive.

d. Related to c. Ask if the beginning of the main story is really the beginning of the story. Maybe the main arc would be strengthened with tertiary action (such as subplots a relative of b.) or with background episodes. It helps to think of a story as a single moment:

Rudgath returned the Sword of De'ark to the Fires of Krynn. (Excuse the cheesy Sword 'n' Sorcery, quick examples require easily graspable story chunks)

Then there is the action that rises into that incident and the action that falls away from it.

Rising: Rudgath retrieved the sword from the Citadel of Unmak.

Falling: The sorcerer Kellior screamed with rage and attempted to murder Rudgath for his actions.

Then you think of events that rise and fall into those two incidents.

A story is a stack of incidents where the primary incident rises into a main incident or "plot point" and then another incident describes the consequence. The story is a tapestry of such incidents woven together to form a kind of conceptual pattern. The 30,000 words of your story form a pattern like this. As the author you can break those words down this way and attempt to weave in extra incident. All of these tips have been about this.

FINAL CAVEAT: What everyone has said about padding is utterly true. Part of being an author is learning to tell the difference between relative moments and irrelevant padding.

William Goldman author of Butch Cassidy and the Sundance Kid and The Princess Bride (among many other movies and novels) talks about a story really being about one thing, a spinal concept, the core of what the author is trying to communicate. Those things that contribute meaning to the core are relevant additions, those that do not are padding.

Hell, it is entirely possible that your current 30k has padding in it.

Screenwriters approach the telling of a story this way (or at least screenwriting courses often encourage them to). The story is a number of flash card "scenes" each of which comments upon the story:

"In this scene we discover that Rudgath is driven to complete his mission because the forces of Kellior slaughtered his brother during the Aeon War."

"In this scene we explore the depravity of the sorcerer Kellioth as he orders seven captured Badger Women of the Sordax Plains to be slaughtered so he can have a fur coat."

As the existence of movies like Transformers 2, X Men 3, the Star Wars prequel trilogy and Terminator: Salvation attest this is not a foolproof method of writing a story but then what is? The one thing this does do is help you gain a view of your story as a communication of tidbits of information which the audience needs in order to make sense of your story, know who to root for, and what outcomes they should want out of the story.

It is entirely possible that your story has information gaps, or places where clarity can be improved. If so, that's the information you want to put in.

You may also find there is "extra" or "irrelevant" information. That has to come out unfortunately.

I guess this is a fascinating and popular question because there are no guarantees. But these are some concrete things you can do to attempt to add real story value to extra wordage and a short guide to things you should be looking out for. Hopefully you, and others find it helpful.