Hi James
This is a great topic to get started and one I feel very strongly about when speaking to friends and family! There are a variety of factors that play into the advice (IE are you do for an upgrade on your account, what's your current device, etc...) But, here are some of my favorite tips & tricks when looking to upgrade:
1. Don't be afraid to buy from authorized reseller like Best Buy or Target. While it may feel weird to not purchase from a corporate store, these stores provide some amazing deals on the highest end flagships. For example, Best Buy is running 200$ off the iPhone 7 right now on a payment plan. That's a pretty big deal over 24 months! Sometimes they run gift cards promotions as well. I usually take the gift card and sell it on cardpool.com and apply the funds to the newly acquired device.
2. Swappa.com is a great place to purchase. It's definitely a buyers site for sure and offers pretty good pricing depending on the seller. The part I like most about Swappa is the IMEI check before someone can list. You won't find yourself in a blacklist situation like ebay. You can also sell your current device on Swappa and use that money to help subsidize the cost of the iPhone 7.
3. Never trade your phone into a carrier. This isn't necessarily specific to the above post, but I just need to get that off my chest for anyone who has ever considered this! While it may be annoying to sell your device yourself, you'll usually get 3-5x's the amount a carrier will offer for your old phone.