45° angles are tricky because of the dimensions of 45-45-90 triangles:
The fact that the hypotenuse needs to be a multiple of radical two makes it difficult to build out of LEGO plates which are generally limited to integers or halves. The best you can do in a reasonably small amount of space is a 5 x 5 x 7.07 triangle, but that doesn't come close enough to work well.
I'm not aware of a way to do this using just plates, but if you are willing to use other bricks as well, there are some ways. For example, some older bricks and most transparent bricks don't include tubes within the bricks. This allows constructions that are not limited by whole numbers:
Edit: There is now a specialized 45° angle plate (15706) that can be used for this purpose: