Photoshop :: Working With LARGE Canvas's, And HIGH DPI?

My questions is this: What should I do cut down the almost frozen status of my computer when Im working with gigantic canvas sizes AND 300DPI?

the project in question is a 6 foot x 3 foot banner, that will be printed out, so the DPI has gotta be 300.....

but when im doing the simplest things, paint bucketing, using the paintbrush, it "loads" for a good few minutes for each move... at this rate, Im going to finish sometime 2 years after the deadline.