In the early days of PCs, anybody with a computer and a good idea could make stacks of money from writing software.

Those days are long gone - unless, that is, you're writing iPhone apps.

There's gold in them there iPhones, it seems. So how do you get it?

We asked three successful developers to spill the beans.

Steve Demeter, Trism

In September, Wired suggested that Steve Demeter's puzzle game Trism would turn over $2 million by the summer. It's a story Demeter has come to regret. "Releasing sales figures was a bit of a novice business move on my part," he says. "I've been swamped with people asking for loans and donations!"

After just two months of Trism sales, Demeter was able to leave his day job writing software for a large bank. "It's been surreal more than anything else," he says. "I had put myself under such pressure to polish and release the game on time, I had burnt myself out two or three times by the time I finished coding. I was running on fumes, exhausted, and numb."

Demeter is quick to agree that part of Trism's success was being in the right place at the right time, although it wasn't all luck - "It's not like my cat walked over the keyboard and out popped a game," he says.

That said, "Trism was the combination of one, getting everything done before launch; two, having a good amount of buzz from people who enjoyed the jailbroken version... and three, Apple getting behind it and helping market it as an iPhone exclusive. Once you have Apple behind you saying 'here, apps like this are why you should buy an iPhone', it's money in the bank."

So has the environment changed? "The market is definitely a lot tougher than it was then," Demeter says. His advice? "Make sure you're asking yourself, 'does my app convey something unique and interesting in ten to fifteen seconds?

"Look at it this way: most people show up to work, or school, or whatever, and they are eager to show their friends what cool new things they've got on their iPhone. Of the 50 apps they may have on their iPhone, they may only get a chance to show five of them to their friends. If your app is one of those five, and it can prove its worth in ten to fifteen seconds, then you've got yourself a successful app."

Marketing matters, too, as Demeter explains. "Start courting an audience before you release," he suggests. "Build relationships with the gaming public and fellow developers. Make sure you can get at least 20 people to buy your game and give you good reviews of your app in the App Store on the very first day it's out. Once that happens, take your app around to review sites. Be polite, even if you get a bad review. And remember to put some of your profit back into advertising - on the web and in-game."

The success of Trism has enabled Demeter to set up development firm Demiforce, which is preparing to launch Onyx, an Xbox Live-style gaming service for the iPhone. "We'll be making an announcement about Onyx soon," he says. "Other than that, I'm just trying to get my life back."

"The one think I really liked about doing Trism was that on top of being a game, it was an opportunity. It was a chance to pull myself out of a day job I didn't like, innovate a game genre and trailblaze an emerging platform, all with limited amounts of time, resources and money.

"Opportunities like this are really interesting - seeing how far I can go with what I have, and showing the world that the determination of an indvidual can make a difference. These opportunities may be on the App Store, they may not be - but you will see me seeking them out for some time to come."

Ethan Nicholas, iShoot

iShoot started life as an after-hours project. "It wouldn't have been so bad if I hadn't already had a full time job," says Nicholas, who was working long hours for Sun Microsystems and coding in his spare time. "I was working eighteen hour days during iShoot's development, but [despite that] it only took six weeks from start to finish - and that includes learning Objective C, Cocoa and OpenGL."

When iShoot took off, Nicholas handed in his notice. So does he spend his days rolling around on piles of money, giggling like a loon? "So far, not much has changed," he says. "I spent the past four years working from home, so I'm still at the same desk, the same computer. Honestly, I don't think the magnitude of the change has really sunk in. I haven't even managed to take any time off yet."

iShoot was initially offered as a paid download, but sales were hardly stellar. When Nicholas introduced the free iShoot Lite, though, sales of the paid app went into orbit. "A picture is worth a thousand words," Nicholas says, handing us a graph showing iShoot sales from its release in October to hitting number one on 11 January. Things got very interesting very quickly when the free version hit the App Store.

Does Nicholas have any advice other than offering a free version to promote your paid apps? "How about 'stop producing so much shovelware'?" he asks. "One number one hit generates more money than one hundred bottom-of-the-barrel apps, so I'd prefer to see more people striving for number one and not polluting the iPhone with so much abominable crap."

For now, Nicholas is continuing to improve iShoot - "online multiplayer support is the next big thing there," he says - but he's got plenty of other irons in the fire. "I'm not ready to share any details, but I have two firm game designs I'd like to get cracking on," he says. "They are very different from iShoot, but I think they both have a shot at number one."

Bill Rappos, iSteam

"Steam simulator" iSteam has racked up more than 1 million downloads to date. Just one week after its launch Great Apps, iSteam's developers, predicted sales of $100,000 per month. We asked Great Apps' Bill Rappos, how's that going?

"The 100 grand was only an estimation we made, based on our first week sales. The way things are right now, it seems we are going to hit that target a couple of weeks later than our initial predictions."

That doesn't mean things are going badly, though. "The success of iSteam has given us the luxury to make long-term plans," Rappos says. "iSteam was just the beginning. We are all really committed to our vision, to become a significant player in the App Store providing a wide range of amusing and cheap apps."

Great Apps is barely three months old, and was founded in November by 24-year-old Kostas Eleftheriou and 22-year-old Vassilis Samolis. They thought the App Store would be a great opportunity, but they didn't know anything about Mac or iPhone development - so they bought a Mac Mini and got to work.

It didn't take long. The team's first "test" iPhone app, Comet Buster, was released in November; iSteam hit the App Store at the end of December. "Coding the initial version of iSteam only took seven days," Rappos says. "One of our project selection criteria is to minimise risks by not spending lots of man hours on a single project."

The team is currently brainstorming its next selection of apps. Does Rappos have any advice for other developers? "There is no easy way in the App Store," he says. "It is saturated with apps, which makes it really hard to survive. Our tip would be, keep it simple and be professional."