Please bear with me!
This section is not as good as I want it to be at all. I would REALLY rather use my own pictures and scans, but seeing as how I don't own a single DBZ DVD or even have a proper scanner, I can't do what I want. Like many of us fans, we have an extensive collection of 18 images on our hard drives. This time, finally, I gave in to using those pictures to make this section better. Some of them are my own scans, some of them are done by a friend, but there are other images I've used where I don't remember where on earth I got them from. That said, if you see a picture you scanned or captured, please let me know so I can either remove it or credit you. Much ♥
Outfit 1: The Red Ribbon (classic outfit)

This is probably 18's most easily recognized outfit and the one she first appears in. Akira Toriyama's characters are never seen wearing any particularly trendy or stylish clothing, but this outfit is pretty slick, I think (fun to cosplay in too!). There's definitely a biker punk feel to it with the boots, denim, and ripped vest (complete with sweet back logo). By far, one of my favorites.
Outfit 2: The Cowgirl (the in-between outfit)

The Cowgirl is not one of 18's favorite outfits, but it was all there was available after her original outfit got mussed up in her fight with Vejita. If I were her, I'd feel icky wearing the whole thing too, but it manages to look pretty damn good on her.
Outfit 3: The ChiChi (raiding-the-closet outfit)

Like her previous outfits, this one is thrown together looking, with colors and accessories that are pretty weird (and sorta clash). As far as her outfits go, this one has a ton of accessories. Why she chose this outfit over the cheongsam in ChiChi's closet, I'll never know, but then again, dresses are useless for fighting.
Outfit 4: The Kame House (pretty in white)

This is my favorite outfit on her. It's so chic and just cute and I would certainly have no problem wearing it. I think this costume only makes a one-time appearance while she's on the Kame House island with Krillin. The whole vest/capri pants theme shows up again. A very classic, casual outfit. ^_^
Outfit 5: The Family Life (return of the orange socks!)

After Number 18 marries Krillin, this is the outfit we see her in most often till the end of the series. She's taken the orange socks and maryjanes she raided from ChiChi's closet and paired them with an identical, but differently colored (read: normal jean color) pair of capri pants. She pulled out her old shirt and belt from her original outfit and voila! And of course, let's not forget the hoop earrings. Sometimes she likes to wear red ones with this outfit, sometimes she likes the old gold ones. Number 18 likes this outfit so much she fights in the Strongest Under the Heaven's tournament with it on. I guess it's a really comfy outfit for her, able to withstand baby spit and the blows of the insane saiyan, Brolli!
Outfit 6: The Business Woman (suits-a-plenty)

I have still not gotten the chance to watch DBGT and see why 18 took to wearing uptight, businesswoman suits all the time. She cuts her hair short to match the look. I like the short hair but not the new look, really. It varies, somewhat, in color and cut, but the typical outfit is pleated pants, a scoopneck blouse, loafers, and a pearl necklace. Sometimes she has a high-cut blazer on, sometimes she is wearing capri pants, but the outfit always looks like it's more suited for a business lunch. I can only assume she started making Krillin dress "nice" too because there are non-stop pictures of him in pleated pants and dress shirts in DBGT. While it is interesting to see the style path she followed in DBGT (which was not written by Akira Toriyama), I would have liked it if she had kept with her simplistic, more easy-going kind of style. And yet...there is still something about these clothes that suit her. :/
Outfit 7: Special Outfit! (the We Got Power/Budokai 2 outfit)

Until Dragonball Z: Budokai 2 came out for Playstation 2, this outfit was unconfirmed, as there were no full body pictures of 18. However, there were a few Japanese DBZ cards showing 18 wearing a plain denim vest with frayed sleeves and plain jeans...but were they capris? And what kind of shoes was she wearing? Was it just a variation on the Kame House outfit or what?? Now that Budokai 2 is out, the details can be confirmed! The question now is if this is the original design of Toriyama sensei or of the game developers? Or perhaps there's just a picture I've never seen...
Other Outfits

In the Trunks' Special especially, and in various trading cards, we get are treated with glimpses of some of Number 18's other outfits. It seems she has a broad, indiscriminate taste in clothing, though it seems to differ slightly in each timeline. In the Trunks' Special, she is seen holding a cheongsam dress before her in the mirror of a clothing store. She smiles at how it looks. But in the series, she decides against wearing ChiChi's Chinese dress. Maybe it was simply because she didn't like the color or something, but I find this little nuance between timelines amusing.
It seems she'll like anything that she thinks looks good on her, in a sort of Japanese streetstyle way, and has no obvious preference to a certain style except a simple, clean one. She of course has a few staples - capris and vests. And denim. She sure likes denim, but then again who doesn't? In the 5th Daizenshuu, there is a little except that says when she dresses casual, she prefers to wear sandals. And what does she wear when she dresses casual? Capris and sleeveless vests! Such a darling. Why did she ever change?
But let's be even sillier for a moment... If I were to try describing her fashion sense by what store she would shops at, I think it would be a cross between Gap, Express and Banana Republic (just think about it, ok?). I don't see 18 as a thrifter or label-fiend. Just someone who appreciates a basic cotton tee (even though we only see her wear one once). All in all, I find her to be, for what series she is in and compared to how everyone else dresses, a pretty stylish lady.