Essentials for sticking to plan:
My Google Calendar - I have everything in it (client meetings, important deadlines, social dinners, birthdays, etc.). With my busy schedule, it's really the only way to keep track of everything!
Lists - I am the queen of lists (I imagine most event planners will tell you the same). I've always been a pen and paper person, so I have keep a notebook with all my lists - by client, by day and, the shortest list, personal tasks (so I don't forget to do things for myself!) I like keeping the same list throughout an event, so I can see what I've accomplished thus far.
Emails - I feel like we're in a day and age that if it's not on e-mail, it doesn't exist; meaning, whenever I do a call or meeting with a client or a vendor, I always like to follow up with an e-mail so everyone is on the same page, and we have it all in writing.