Plugging in a USB cable is often an exercise in frustration, involving multiple jiggling and flipping attempts when really this should be a straightforward connection. Avoid this annoyance going forward: Look to the USB connector's seam.
We've mentioned before that the USB symbol on cables can tell you which way is up and thus help you plug it in the right way every time, but sometimes that symbol isn't there (e.g., on a flash drive).
In lieu of making your own marker on the cable or drive to note which way to plug it in, look for something that is on every USB connector: the seam. Lifehacker reader coren tipped us off to this. The seam should be facing down when you plug the USB cable or flash drive into a horizontal port.
Apartment Therapy confirms this and says that for vertically oriented ports, the seam should be facing left.
So, remember, seams down or left, and goodbye USB frustrations. Update: the actual orientation might not be down and left for your system, as some commenters have noted (e.g., if you have a cheap case). But, once you know how your system's USB ports are aligned with your USB cables' seams, then you know how to plug it in every time going forward.
Clever USB Connection Solutions and Tips | Apartment Therapy