I've used different methods for marking up logs before cutting into consistent lengths, but those methods proved to be time consuming and not very accurate. Until now:
acc-u-mark.com
It came today, and I was able to use it a little. It's extremely well crafted, light weight, simple and easy to use. Not only do I get the correct lenth of wood, but it also helps me to start a square cut as well. I highly recommend this tool

ACC-U-MARK FIREWOOD MARKING TOOLS
Pro-quality firewood marking tools for professionals and DIY firewood producers. Instant on/off, no tools, simple and accurate. Made in Montana.

It came today, and I was able to use it a little. It's extremely well crafted, light weight, simple and easy to use. Not only do I get the correct lenth of wood, but it also helps me to start a square cut as well. I highly recommend this tool