Good Sammy's in Joondalup - one of the cheapest, quietest, and best organised second stores going. It's just out from the Centre on Grand Boulevard. Salvos at Whitfords Shopping Centre is also super cheap.
In the city, there's the renowned RetroVinnies in Northbridge, but don't make the mistake of paying extra for people to find the best stuff for you; both Salvos and Good Sammy's on Hay street have ample of 'retro' without the prices to match.
Keep your eyes out for the monthly We Heart Vintage sale in Mt Lawley, and the less frequent Polka-Dot Vintage sale in Claremont Showgrounds, the latter also sporting weird and wonderful antiques, (entry usually only $6). This is your means to suede skirts, shoes and shorts, so don't miss it.
For those with a bit more cash to throw around, try Oh Henry on Williams Street, or Blue Flamingo in Subi for some pre-picked, excellent quality vintage gear.
Remember; just because it's old, doesn't mean it's trendy - pick things you like, don't try too hard, and enjoy some of the wonderful pieces out there.