Have a look here:
Each of the two USB ports on the Pi has a polyfuse rated at 140 mA, so
any connected USB devices should draw less than this amount of
current. In addition the polyfuse will cause a significant voltage
drop, so that USB devices get less voltage than is available on the
RPI itself, sometimes up to half a volt less (maybe more if the fuse
has recently been hot).
In other words, the pi is not designed to power anything significant from the USB ports -- technically a USB port should provide 500mA, but the pi's are limited to 140.
So, get a powered usb hub. If you get the right one, you can power the pi itself too; while the USB standard dictates 500 mA per port (no less and no more), some hubs do not regulate this and will output whatever up to their total amperage rating. I had a look through here last week for one people reported as useful toward this purpose, and can confirm that the "Belkin F5U237" 3.8A 7 port hub does not seem to have any problems! It was easy enough to find locally but cost $30; there are smaller ones on the list that apparently work too. I would assume if Belkin don't regulate one model, they probably don't regulate any of them.
It's nice having the upward facing ports to smack a keyboard into occasionally too.