Cut your downhaul costs in half (or more than cover the costs of upgrading to Spectra)!
You already may do this and find this stupendously obvious, but most people I mention this to had never thought of it before, including a pro windsurfer whose jaw dropped when I explained it: You can get twice the life out of a downhaul line by flipping it.
It's simple: The only section of a downhaul line that wears out is also the only same section that needs to be able to handle hundreds of pounds of tension: the first 12-18". You can remove a downhaul line that's starting to show excessive wear (photo, left) from your mast extension, cut off the knot, flip the line, and reattach it to your extension, and now the portion of the downhaul that's running over the pulleys and going into the cleat when you're fully downhauled is in nearly brand new condition (photo, right). Now each downhaul can be used twice as long.
If you're particularly conservation-minded, cheap, poor, or otherwise enthusiastic about such strategies, there's more. Once your downhaul line is worn at both ends, you can use the four good feet in the middle for an outhaul, where the stresses are a fraction of what they are to the downhaul or inhaul. Don't forget to check your inhauls while you're at it! And even the little bits of safety retaining line on your unis, if they use them. Having line break on the water is a drag, if not downright dangerous depending on which line, the conditions you're sailing in, and where you are.