UPDATE 9/29/14: This tutorial no longer works. Apple has stopped signing iOS 7.1.2. Instead iTunes will present you with an error message.
If upgrading to iOS 8 has you down, don’t worry, you can still go back to the stable environment of iOS 7.1.2 (the last version of iOS 7). Just don’t wait too long, as there is no telling when Apple will stop supporting iOS 7, which could make downgrading much more difficult or even impossible.
WARNING: Before you start the downgrade, you should back up your device so you have a fallback in case your data goes missing or anything goes wrong. iOS 8 still has some issues (otherwise why would you be here?), so it can respond in unpredictable ways.
These steps are the same whether you have a Mac or Windows PC.
Step 1. Download the iOS 7.1.2 version that corresponds to your device and save it to an easy-to-find place like the desktop. You can find the right firmware for your device from this list via OSX Daily.
Step 2. Open iTunes and connect your device to your computer via USB.
Step 3. Select your device in iTunes from the tab in the upper right corner (just be careful not to hit the triangular eject button.
Step 4. Go to the summary page.
Step 5. Click "Update" while holding the Option key on a Mac or Shift in Windows.
Step 6. Select the firmware from the navigation menu and confirm you want to proceed.
Step 7. Hit update again. iTunes will check to see if the firmware is valid and then move on to the downgrade.
When the device reboots you should be running iOS 7.
Check back with Tom's Guide for updates and to see when it's finally safe to go back to iOS 8.