Airline ticket buying

I wish I knew the secret to buying cheap flights on the internet. It seems that every time I need to fly somewhere, the tickets are always really expensive. But when Kevin looks for flights, he always gets these great deals. So what gives? I try and ask him for help and for secret knowledge but I still can find a flight from PDX to CHI for less than $300 in July. Kevin found our flight to Chicago in May for $178. So unfair! One minute the flight is cheap, you click to buy and the flight is no longer available or hundreds of dollars more expensive. It's so frustrating! I would appreciate any tips out there for finding cheap flights. I need to get to Chicago in July for a week. Right now the flights are $300 and counting...