Dont stretch the cable over 300 feet, thats a max with quality cable. If its near that length (with both cables) or longer, use a powered hub or switch to connect the lines, it will act as a repeater for the signal.
just a note on using a coupler though, most will drop the rating from 100bt to 10bt, check for this as well as making sure its not a crossover coupler like Ladi stated.