Q: Will I get better results if I use the same brand of shampoo and conditioner rather than mix brands?
A: I put your question to my e-mail friend (and cosmetic chemist) Mort Westman of Westman Associates in Oak Brook, Illinois, who responded, "During my 30-plus years of working with hair products, I have never encountered companion shampoo and conditioner that produced a perceivable synergy when used in combination." Meaning: No, you will not get better results if you stick with one brand. Mort being Mort, though (always striving to paint the full picture), he added that if you do choose to use products from the same line, you might be sparing yourself what he called "fragrance conflict"—and you'll benefit from the aesthetic bonus of matching containers in your bathroom. But you should feel free to switch products without any adverse effects.
Keep in mind: Here's a related tidbit I picked up recently. You might find, if you read the fine print on the back of a shampoo or conditioner bottle, that the performance claims on the front ("strengthens hair," for example) are actually based not on the results of using that one product but on the cumulative results of using a regimen of shampoo, conditioner, and possibly other products from the same line. Caveat emptor!