Does anyone have a good strategy for maximizing return on excess cash?

I hate to leave my excess cash in my investment account earning zero, but I also hate the loss of liquidity by transferring it out. I think the cash mutual funds have minimum investment periods, but am not sure.

I am looking for something highly liquid (next day or better access), has return greater than 0% for investing and redeeming within 1 week, and has principal protection.

Any suggestions?