Depends on what I'm looking for, and what brand.
The only time I ever go "brand-whore" is in regards to Lolita clothing, and even then I have a set of rules I always follow:
1. It has to be versatile; that is, able to be worn in several ways. This means a jumperskirt instead of a dress, or something in a neutral color.
2. It needs to be something in a print or style that I could not make myself. Generic florals or solid fabrics are out, but I'll spend money on something with embroidery I couldn't recreate or a cotton print specific to the brand.
3. It can't be more than $300.
4. It has to be a brand that I trust, and if it's my first time buying, I do massive amounts of research and asking around before committing to anything.