Color and Contrast
First of all, this is the simplest way to begin building a transmog. What is your favourite color? You could opt to build a transmog set around that. For the transmog above, I set out to make a transmog that had a lot of pinky purples and blues. You're going to want the addon MogIt. This is an invaluable tool for keeping track of what you own and filtering potential items in various ways. It'll make your transmogging much, much easier.
Suppose you've decided you want to make an outfit that's orange. You'd go to MogIt, select the "Cloth" module. Then you'd look under "Catalogue" and "Sorting," and then "Approximate Color" to choose a shade of orange that appeals to you. I often start with a robe to build a set around. Depending on the color you might have chosen, this can present you with many options. Feel free to just focus on things you already have. Every transmog in this article uses items I already had (granted, my closet is...fairly extensive).
The next consideration when you're mapping out a transmog, after you've chosen a color or colors to focus on, is contrast. You don't want to be a beacon of unremitting orange. Look at what other colors are in the set and consider what you could mix and match. What do you want to call attention to most? You might have a really stellar robe. Match it with some more understated gloves, shoulders and belt to make sure it takes center stage.
Remember too that you can play with different tones of colors. There's a difference between bright tomato red and an understated oxblood. You can have the brilliant orange of a pyroblast or a paler, muted beige. I won't get too heavy into color theory on you, here, but some basic knowledge of how color works can really help when you're making a transmog. You can't go too far wrong by focusing on a monochromatic scheme. Also, colors that are next to each other on the color wheel will always be harmonious (e.g. red, red orange, orange). You'll find that many sets in WoW are already designed with a particular color scheme in mind. But rather than using the entire set, you can find matching pieces that echo the colors used.
You might want to let a particular piece of gear that you love determine the rest of your transmog. A good example of this is my transmog below. I am a huge fan of the Apostle of Argus (staff) as pictured. It is so attention grabbing and has a draenic flavor, so I built the rest of the set with that staff as the centerpiece. The main pieces of gear are understated greys with small hits of the same electric blue that the Apostle has. The resulting outfit lets the blues catch the eye while still giving a cohesive feel. And it's not just an entire tier set!
Apostle of Argus Set
Head: (Not shown) Corp'rethar Ceremonial Crown would look good though, or Chronomancer Hood to match the set
Shoulders: Iceshear Mantle
Chest: Chronomancer Robes
Hands: Static Charge Handwraps
Belt: Belt of Divine Guidance
Weapon: Apostle of Argus
But what if you really like a particular tier set? That doesn't mean you can't use it. If you want to break the mold a little, though, take 1-2 pieces from the set and see what else you can match it with. Let's use Tirisfal Regalia as an example, because it's such a popular set, in particular the hood.
Imperial Fire Set
Head: Cowl of Tirisfal
Shoulders: Mantle of the Fatigued Sage
Chest: Imperial Ghostbinder's Robes
Hands: Crafted Malevolent Gladiator's Silk Handguards
Belt: Fire-Cord of the Magus
Weapon: Ti'tahk, the Steps of Time
This is an example of how you can take one iconic piece and really make it your own. It also conveniently leads into the next consideration - thematic sets. Even though these pieces aren't all mage specific, it's still pretty clear that this is a fire mage set. If you wanted to be more subtle, you could choose a less fiery staff and let the deep reds speak for themselves.
Transmog is such a great way to express your personality and really customize your character. You could focus on making a set specific to your spec of choice or choose a more esoteric theme. Use your imagination, you're only limited by what's available in game (and what you can get your hands on, of course).
Head: Starfire Tiara
Shoulders: Khadgar's Shoulderpads of Conquest
Chest: Ebonweave Robe
Hands: Static Charge Handwraps
Belt: Cataclysmic Gladiator's Cord of Cruelty
Weapon: Starshard Edge
Here's an outfit I made based around the idea of a "celestial" transmog. It uses a few key pieces from Ulduar to pull it together. I also used the Wonder Woman lookalike Starfire Tiara as a nod to the star theme.
These are just a few examples I have put together to show you what I mean, but really there is so much you can do with transmog. Maybe you want to try to look like a mage from Final Fantasy - there's a hat for that! You could use some of the Sha-inspired gear from the last few tiers to put together a really fearsome Sha set, or look like a Highborne mage or a Blood Elf straight out of Magister's Terrace or Sunwell.
Head: Bloodwoven Mask
Shoulders: Mantle of the Tempest
Chest: Evoker's Silk Raiment
Hands: Gloves of Tirisfal or Gloves of Arcane Acuity (I would've used these if I had them)
Belt: Captain Sanders' Sash
Weapon: Merciless Gladiator's War Staff or Crystalheart Pulse Staff
This last set is just an example of another thematic option, my idea of what a battle mage set could look like. It has a bit of a draenic influence (shocking, I know). It also brings its own illumination so you don't need a booklight when you need to hit the books!
What about making a set that takes everything mentioned above - a standout item, a color scheme, and a theme, and make it a spec-specific transmog? Expand what you think about each mage spec to really make it your own. Frost mage transmogs are usually based around blue and white. But what about a wider consideration of what evokes "frost"? Deep glacial aquas or the dark frozen water of a lake in winter could also be a starting point for a great transmog that's unique. Of course the same applies to either of the other specs. Arcane especially has the most leeway - any shade of purple is instantly a great arcane transmog, but what color is "magic" really? It could also be prismatic or white. Blacks, reds, oranges, yellows or any combination of the above could work equally well for fire.
I hope this gives you some ideas for expanding your personal fashion concept with transmog. I'm not saying that an entire tier set is bad - sometimes I wear one, too, because it just looks so nice and cohesive - but don't be afraid to experiment, either! The only thing better than being a mage is being well-dressed while you launch a volley of deadly magic at someone's face. There are many transmog resources right here at WoW Insider for you to check out if you want more ideas.
Now I have to deliver some sad news in case you missed it. This will be the last Arcane Brilliance I write here at WoW Insider. I wanted to thank you all for being so welcoming from the very beginning. Although I've only been here six months, it's been a pleasure to write for you and engage with the mage community at WI. You are excellent people, great killers of warlocks and other annoyances. I wanted to go out on a high note, and I love talking about transmog. If you'd like to read more from me, I'll still be blogging at my personal mage blog, Manalicious. I'm also pretty active on Twitter here: @_vidyala. May your spells never miss and your conjurations always result in the best confectioneries. It's been a slice (of delicious cake).