Thankfully the liver is quite resilient and can recover from minor stress while being on cycle. Most everyone knows that oral steroids are nutorious for causing liver stress. They are best used for specific periods, such as kick starting a longer, injectable cycle or for the final weeks of a contest prep, to help acheive a harder, drier look. Injectables are safer but they are also going to tax your liver. Your best bet is to be in the best possible health when you start a cycle, limit your use of orals and take time off between cycles to let your body regenerate. When I talk about "off" periods to recover, I'm not only talking about recovering your natural test production, but also letting your liver heal, letting your cholesterol go back to normal, letting your RBC go back down, ect. These things can still be done for the guys who use HRT between cycles, so anyone out there that isn't planning on recovering their natty test, be aware, test is only one of many functions that need to be normalized after a cycle is done. If you take care of yourself and find balance, you will have a better chance of staying healthy. Your blood work will let you know when you are ready for your next run. 

In short, there is no supp that you can take that is going to protect you. There are a few that you can look into using that may help, but the effect they have is minor. By no means will it save you, IMO. I would look into Milk Thistle, N-acetyl cysteine, and Liv-52. These are 3 power houses of liver health. I run milk thistle for long periods at 1000mg ED. I use it on and off cycle. I use N-acetyl cysteine during my contest prep and I add in liv-52 while using orals.