Are you still spacing twice after each sentence like we learned in 8th grade typing? Well, stop. Now. If you're writing professionally, this practice will brand you as novice. It's a little thing, but it turns into a big problem when you repeat this error throughout a manuscript.
The reason for double spacing was that, in the early days of typing, each letter of the font took up the same amount of space, so we needed two spaces after a period to be better able to recognize where the sentence ended. Now we have proportional-spacing fonts (each letter taking up only the amount of space it needs) so one space at the end does the job just fine.