In addition, try to make sure you're not using a 'front' USB port. Oftentimes, the mobo manufacturer's will skimp on power output to 'accessory' USB ports, whereas the ones going directly to the motherboard via the back panel will get the benefit of the full 5V of voltage with required Amperage.
But Zig's recommendation is correct - a powered USB HUB is the only way to fly. I personally have one of these: CPH420P - USB Hubs | CyberPower Systems, Inc.
- that I picked up at a local Wal-Mart (in the US) for ~$20 USD over 2 years ago. Still kicking, and never giving me issues.