i have a folder similar to yours. it never leaves the house though. i have a notebook for customers in it. i put their name, address, phone number, how often i cut and what i charge under their name. i have another notebook i keep track of expenses in. i have another notebook in there i keep track of when i was paid and how much and who paid me. i update things every day. i don't put it off and let it build up. if i get paid i then go check it off on the calendar i have. i buy something i write it down as soon as i get home. i keep all my receipts in a shoebox. it takes less than 5min a day to keep everything updated daily.
as far as keeping track of schedule i use one of those big desktop calendar things. i write down what i'm doing on each day and i check it off when done and i check it off again when i'm paid for that job. makes it easy to keep up with everything. i keep a seperate notepad in the truck in the glovebox to write down addresses and such when people call me while i'm out working.
i keep every customers name and number in my iphone as well. you could also use the calendar on the iphone if you wanted to but i don't do that at this time. the desktop calendar works for me as 5 yards is a full 8hr day here. sometimes 3 yards is an 8hr day. some of them are 3acres in size. the days you cut those you many only cut 2-3 yards that day. if you cut small yards all day then you may not have the space to fit 10 customers names in one box on the calendar lol.
i don't get all complicated with it. i don't use fancy computer software or anything. i'm solo so it's overkill to do that. if you have a big company then it's probably necessary in order to keep track of everything.