My suggestion is a rather universal one: as the GM, you always have to roleplay every character. Make each person at least a little interesting not by changing up stats or equipment, powers or spells, but rather by giving them life. Joe the security guard, for instance, might not really want to fight the runners. Maybe he's had a really shit day and just wants to go home. So he surrenders or lets them slide, or maybe he secretly wishes he could be a runner and asks the team for help getting out from the corporate yoke. I'm not saying this should be the norm, because if it was there'd be no security guards and no shadowrunners. But what I'm saying is that you can make the world feel more alive by thinking about what makes people come alive: personality.
Lots of people like to use props, music, mood lighting, etc. That's all fine and good, but it won't make up for a lack of great roleplaying and thoughtful storytelling. Keep the world realistic by thinking like the people in the world. If the runners are hired to steal an item from a location, you have to think of all the angles: why does Mr. Johnson want the item? Who hired Mr. Johnson? Does Mr. Johnson intend to use the team as a distraction? Is Mr. Johnson looking to establish a long-term arrangement with a team? Who built the security system, and for what purpose? Would someone have put a back door in some aspect of the security (physical, Matrix, magical), and if so why? Who are all the parties involved in this deal - maybe there's someone else who wants the item but doesn't want to deal with the security, but could deal with a team focused on different skills?
What I did with my campaign is find a ton of pictures that could inspire me. These didn't have to be Shadowrun-specific, but they did have to convey a sense of character. Then I fleshed these people out as much as I could with just basic stuff - name, age, role. I tried to create an infrastructure that could be utilized, building a few different fixers, a bunch of Johnsons, some arms dealers, a few gang members, a few syndicate members, a bunch of street docs, etc. These are all the types of people that would benefit from a shadow economy. Finally, I added a layer of NPCs that were relatively peripheral to the shadow economy, people who were incidental. These people either rely on the others in the economy, or they somehow assist it without being "important." Hookers, pimps, drug dealers, hackers, forgers, pawn shop owners, junkies, street kids, gangbangers, wannabes, etc.
I even have an NPC that is a 12 year-old kid, living on the streets, using a bare-bones cyberdeck. He was a hacker before the change in the Matrix protocols, but he managed to steal a cheap low-level deck. It's more than sufficient for him to hack vending machines for clothing, food, etc. He hangs out at a skate park with some runaways and stuff. He has his ear to the ground, but he's not all that woo-woo. Still, he's fully developed and pretty cool.
Once I had all these NPCs - there's a ton of them - I had to build some locations for them to live in. So I began with places where the important characters would meet up, likely be located, etc. Every criminal has some sort of haunt - these were the haunts for the NPCs. Favored locations for meets would be fine too, especially for Johnsons and fixers. I started out by using any locations that seemed reasonable from Seattle 2072, a book of immeasurable value. When I couldn't find quite what I was looking for, I began to create my own locations - just names and addresses, really. Using Yelp and Google to find location names and addresses from in and around Seattle helped a lot.
For instance, I have an NPC fixer named Paradox. He's an orc, and he's smart as hell and very charismatic. Hence the nickname. He was the last son of a minor British noble family, but he was ostracized for being an orc. Despite all that, he set out to create a name for himself and he knew than an education would be the fundamentals. He grabbed what he could before he got kicked out of the family. After being kicked out, he managed to get in good with a mentor of sorts - a guy that owned an import/export business. He decided this would be a great business to get into, and moved to Seattle - it's far from his family, and there's no shortage of goods being shipped in and out. Now he owns a similar business in Tacoma, but it's a front for his real money-making business: being a fixer. He'll occasionally let someone move illegal goods through his warehouse, but the goods are always gone within 24 hours. For fixer-related stuff, his business is to make friends and make connections. He's very good at that. Technically, the information he has could put a LOT of people away for a long time, but he hasn't actually ever done anything illegal. He's just helped facilitate some friends meeting other friends. On his office door, it just reads "Boss" - and he has a very pretty orc secretary that he sexually harasses a lot. He might be nobility, but he doesn't have to be noble.
