So lately I've seen a lot of posts on the boards about "game balance is the worst its been in awhile", "game sucks now" and nostalgia threads about seasons 1-5. If you feel like the game is taking a turn for the worst, then make your voice known by hitting Riot where it hurts the most, in the wallet.
Simply don't buy anything from Riot while you feel like the game is in a bad state. Eventually, they'll get the message and hopefully the game can get back on track.
Edit: Since several of you now have suggested that voting with our wallets might not work, and that not buying RP/skins does not equal having problems with the balance of the game, (valid point) what would you suggest we do instead, seeing as our posts on the boards don't have as much of an impact on Riot's balance/gameplay decisions as they should.