A particle going at speed v has energy given by:
E = mc<sup>2</sup>/sqrt(1-(v/c)<sup>2</sup>)
As v approaches c from below, E gets larger and larger, until it is infinite when v=c. It is impossible for an object to ever accelerate to the speed of light because it has to get its energy from somewhere, and there are no infinite supplies of energy.
BUT...
There are theoretical particles called <b>Tachyons</b> which always move faster than the speed of light. Consider the energy equation with v>c. Then, the square root is the square root of a negative number, which has an imaginary value. To make the energy real, tachyons must have imaginary mass. (Note: the word "imaginary" here is used in its technical, mathematical sense.)
A tachyonic particle with mass im (i=sqrt(-1)) would have energy:
E = mc<sup>2</sup>/sqrt((v/c)<sup>2</sup> - 1)
This energy decreases as v increases. The energy is zero when v=infinite. The energy is infinite when v=c.
This means that tachyons would always travel faster than the speed of light. To slow a tachyon down to the speed of light would require infinite energy.
The important thing to remember, though, is that nobody has ever detected a tachyon. Such particles are purely theoretical for now. We don't know what something with imaginary mass would look like.
It's an interesting possibility, though!