Do these tasks to get new clothes.

Effect - Code

Army Fatigues - Beat the mission "Last Stand"
Casual Clothes - Beat the mission "Conduct Unbecoming"
Cuban Clothes - Beat all of Umberto's missions
Hired Muscle Clothes - Purchase a "High Roller" Protection Racket building
Hood Clothes - Purchase a "High Roller" Robbery building
Leisure Clothes - Purchase a "High Roller" Prostitution building
Mr. Repo Clothes - Purchase a "High Roller" Loan Shark building
Smuggler Clothes - Purchase a "High Roller" Smuggling building
Track Suit - Purchase a "High Roller" Drug Dealer building
Trailer Trash Clothes - Beat the mission "D.I.V.O.R.C.E"
Wetsuit - Complete all 8 Jetski races at the Film Studio