Sunset Overdrive

To earn this achievement, you will need to acquire just about every fashion item in the game.
You get fashion items from
- Story Missions
- Side Quests
- Earning Gold on Challenges
- Buying them with $$$ from Calista
- Collecting all of each collectible generally rewards one or more items.
- Chaos Squad
- Update - There is a lot of clothing that can be unlocked through Chaos Squad. You need to be at least halfway decent at the game though to get the later roullette boxes. Clothing can be in any box but it's much more likely in the later ones, as the first ones are generally weapon boosts, money, or overdrive.

I earned this with 3 items left from Calista. I had earned 1 item in multiplayer.
Things to do while working towards this.
- Complete the Story
----- Many missions award clothing and you'll be doing the story anyway for achievements.
- Complete every Side Mission
----- Same as Story, you'll earn achievements for completed groups of related quests
- Gold Medal every Challenge
----- You need to do at least 50/54 at Gold for other Achievements
- Buy everything from Calista
----- Some of her stuff is ridiculously expensive, I'd suggest starting with the cheapest.
----- I did this and only had about 5 ridiculously expensive hats left at the end of the game
----- I did traversal challenge #13 a bunch of times and it gives 10k each
- Depending on how much clothes are available from multiplayer you may not need to do all this. *Update - I have gotten A LOT of clothes from multiplayer.
----- I've gotten clothing from every box, but its much more likely with the later boxes.
----- You'll most likely have to be in the game from the start, and you'll need bonus objectives.
----- Every bonus objective gives a 2x multiplier to the score, and then you get the chaos % added.
- If DLC clothing counts as commenters say that lessens the need to work on harder achievements just to get this.

The main difficult part I could see people having trouble with for this achievement is the Gold Challenges.
The hardest ones (for me personally) were the 4 for Buck, where you have to do various optional objectives to earn score.
My advice would be do a round or 2 of the objectives, and then just go crazy with the teddy launcher. Earning a 'kill streak' earns you 10k points, and you can rack up a lot of these if you just annihilate everyone.
I'd say more but that would probably be more relevant to other achievements.