Figure Out if You're Going to Trade-In Or Sell
This is a good time to figure out what you are going to do with your old phone. You can trade-in a phone at Best Buy, Target and at any carrier that sells the Galaxy S8. You should search for the best price since the prices will likely vary greatly.
If you trade-in your Galaxy S6, Galaxy S7 or iPhone, you will get less than selling it directly, but you avoid the hassles of shipping or meeting someone and there are some times where you can score a higher price when trading in and upgrading.
You can lock in a trade-in price so that you get a good price and can keep using your phone until the Galaxy S8 release date in late April.