It's an interesting question, particularly because you cannot avoid multitasking in marketing projects because you cannot push a string.
That is, prospects are going to take a certain amount of calendar time to buy and you can't wait for them to do that or ride their ass to speed them up past a certain point.
For a high-touch sales process, customer relationship management is a high art, practiced even by lone wolf salesmen who put up big numbers. For other kinds of marketing I use the issue tracker in one of my github repositories, although I can't say it is a great system or I use it well.
I think the core thing is keeping track of commitments you have made. It might be "post a blog post every week" or "call five prospects every morning", "answer customer inquiries in 24 hours", "call Mary at the sign shop to replace the sign in front of the store, etc. Note some of these tasks are periodic, some of them are a matter of responding to events, others are specific one-time events.