Lifehacker Reader Jared Valentine tried ensuring a fast internet connection by setting Quality of Service rules on his router but found that QoS just wasn't working well for his needs. To solve his problem, Jared wrote an extensive howto detailing how to set up his computer to automatically detect when he's on a VoIP call, then aggressively limit his other traffic to make sure he's got plenty of bandwidth for his voice calls. The tutorial is not for the faint of heart, but if you can pull it off, Jared calls it his holy grail of traffic prioritization. If you're looking for a simpler way, you may want to give router QoS rules a try to see if they'll work for you first.
Application Aware Triggered Quality of Service (AATQoS)