How to make registration/login for premium account purchase as painless as possible?

I am currently working on a fundraiser to gain several thousand premium accounts subscribers for my site.

Because these are premium accounts (like Reddit Gold), a user must have an account so the payment can be linked to their account.

The process would be register/login first (via AJAX, so no page reload), and then fill in payment information. However, I have repeatedly seen that users don't like the registration/login requirement (ex. $300 million dollar button).

Is there a way to make the process for purchasing a premium account as painless as possible? I'm unaware of any way to remove the register/login requirement, because the user needs an account to get a premium account.