What is a better way to present sign up / login form?

It has become quite common and dare I say annoying these days to see websites and mobile apps prominently highlighting the 'sign up' for new user fields but at the same time why do they make 'Login' for existing users so hard to find?

I understand that these companies want to get more users onboard, but it just makes a poor experience for existing users, case in point - try to find the login button in this screenshot taken from https://www.pinterest.com home screen, it took me a while to locate it.

So the question is, how would you make sign-on for existing users better without creating a barrier for new users to sign-up?