To get the following level bonuses, you have to complete the corresponding task:
Level 1 "Warehouse":
Defeat 20 enemies: Flaming Beanie
Get to your car: Freeze's Starting Outfit
Take two Human Shields: 25 To Life Decals
Level 2 "Apartment":
Cover up 4 tags: Black Bandanna
Kill 25 enemies: Barbed Wire Hat
Return to you car: Rick Ta Life Outfit
Level 3 "Downtown":
Break into ATMs: Gold Earrings
Complete level without dying: White Sweatband
Defeat 25 Enemies: Westside Knife Tat
Escape to your getaway car: Bank Robbing Outfit
Level 4 "Culdesac":
Get 20% Accuracy: Holster
Apprehend 5 felons: Kneepads
Get the evidence to Mendoza's car: Officer Williams
Keep your partner alive: Tactical Vest
Level 5 "Club":
Achieve 25% Accuracy: Big Mustache
Apprehend 4 drug dealers: ATF Decals
Get to the subway: DJ Envy Outfit
Register 8 Head Shots: Riot Helmet
Level 6 "Subway":
Achieve 30% Accuracy: Big Dawg Tat
Defeat 24 enemies: Sideburns
Free a cop: Full Tac Harness
Trap Shaun in subway train: Shaun's Getaway Outfit
Level 7 "Tijuana":
Kill 60 enemies: Clock Cap Decal
Find Riggs at the club: Shaun's TJ Outfit
Tag over 4 gang tags: Mexican Flag Bandanna
Level 8 "Casino":
Kill 45 enemies: Oozie Chest Tat
Escape with loot: Federates Outfit
Register 12 headshots: Scorpion Black Tat
Rob five cash carts: Triple Bracelets
Level 9 "Penthouse":
Defeat 32 enemies: Camo Pants
Register 16 headshots: Chest Flame Tat
Level 10 "Prison":
Complete Level: Freeze's Prison Outfit
Kill 30 enemies: No Parole Decal
Protect Lazarus: Chinese Pain Tat
Level 11 "Mall":
Break into 5 ATMs: Black Gloves
Finish level without dying: Red Hightops
Complete level: Freeze's Mall Outfit
Level 12 "Hacienda":
Get 5 secret stashes: Bulldog Chain
Defeat 30 Enemies: G Cap Decal
Defeat Shaun: Daz and Tech N9ne