Give me a strategy for getting to 500% multiplier and/or donut farming?

I don't know a number.
But I'm at 800%, and I can now buy everything that is released in the store (either new stuff, rebates on existing stuff, or re-releases). But I still don't have a lot of stuff that is always for sale at the store (Barney for example).
So, I plan to keep increasing it for now.

(besides buying stuff, you might also want donuts to rush tasks during events, which I'm not considering here)