Practical Idea No. 1
Buy From a Sustainably Responsible ManufacturerHarvesting wood for all types of musical instruments is big business, and the short-sighted deforestation of woods in the old days lead to the extinction of highly demanded "musical" woods such as Brazilian Rosewood and Adirondack Spruce. Since then, laws have been put in place that force manufacturers to adhere to different wood-sourcing practices, but certain guitar builders go above and beyond.
Doing a quick google search of your favorite guitar maker's wood-sourcing eco-footprint will yield some interesting and innovative practices, but Taylor Guitars has been at the forefront of environmental ethics. One example of their business model is partnering with small villages in emerging countries such as Honduras and Cameroon, where they partner different villages together and give them the machinery and training they need to source their own wood, vitalizing the community with a skilled labor force and bringing modern efficiencies of cutting and regrowing a forest.
Impractical Idea No. 1Choose to have an Earth-friendly cremation instead of a wooden coffin burial after you grow old and die waiting for the new Tool album.
Practical Idea No. 2
Get More Life Out of Your StringsQuit being so cheap and just buy the Elixirs. They really do last at least 3x as long in my experience, they're easier on your hands, and in a lot of cases they sound better (I vote for the phosphor bronze ones, just check out some YouTube shootouts for proof).
Regardless of what strings you use, the dirt, oils and sweat that everyone accumulates on their hands will damage and corrode your strings over time, thus shortening their useful life. This can be nullified with the mind-blowing modern technology of WASHING YOUR HANDS before playing.
Impractical Idea No. 2Turn off the light and practice your Lamb of God riffs by candlelight, the way they were meant to be played.
Practical Idea No. 3
Stop Printing StuffIt started out with good intentions, but when is the last time you even opened that enormous binder stuffed with Bon Jovi tabs and chord charts? There are so many awesome guitar resources that can be acquired as E-Books or downloads. A lot of them are even free, or at least cheaper and can be stored on a phone or tablet.
Impractical Idea No. 3Carpool in your bass player's car to band practice. Why is this impractical? Because bass players don't own cars.
Practical Idea No. 4
Buy UsedSecond-hand gear is essentially the recycling of the gear world. In most cases, used gear is cheaper and just as good if not better, so you're doing the planet and your pocketbook a disservice by not at least looking into buying used. Pawn shops can be kind of risky, and Craigslist could get you murdered so if you don't feel like rolling the dice, check out gear from longtime preferred sellers on eBay or Reverb.com.
Impractical Idea No. 4Stop using electricity to power your pedalboard and instead use tiny, tea light candles... Oh wait:
Practical Idea No. 5
Use Rechargeable BatteriesThis is something else that is better for the environment and will quickly end up paying for itself anyway. You can get a 4 pack of Duracell 9 volts for your pedals or onboard electronics for about $10. Or for just over twice that, you can grab a 4 pack of rechargeable lithium-ion 9 volts with the charger. Same goes for AA and AAA, after 1 or 2 charges it pays for itself.
Impractical Idea No. 5Instead of making your own new songs, you can reduce waste by reusing and recycling riffs from a lesser known band that you toured with in the late '60s, leading to a long, drawn-out lawsuit, that probably won't matter anyways, because your recycled riff is way more awesome and popular than the organic original.
Another practical way to be an Earth-friendly guitarist would be to join the #AppsForEarth campaign and get a useful guitar app while helping a noble cause. You can do that here.