Yesterday we talked about the fact that readers form most of the images in their head not from the words on the page, but from their own imagination. As an author, you can take advantage of this by giving just enough information to let the reader see what they need to, but not so much that you slow the story down.
Brandon Mull wrote the popular Fablehaven series, which featured a brother and sister. I regularly ask readers if they can see these characters in their head. They all can, despite the fact that fact that Mull never gives a detailed description of their looks.
Instead of telling me your main character has shoulder length, carrot-red hair, a bony build, is 5'7" and has lots of freckles, tell me she is an athletic redhead and move on to her other characteristics. Focus on what you can do best in writing, which is to show us what's inside the character and let the reader form their own image of the outside.