Take a quick survey of the alpha-range you're shelf-reading so you don't inadvertently jump to the next subject letter. I usually mark the first and last books in the range by pulling them out halfway. Remember, "LCC tunnel vision" often happens when focusing on Cutters and losing sight of the entire call number.
Shelf-reading is detail work. It is more important that you take your time to be accurate than to rush through quickly and make mistakes.
Take breaks as necessary!