We've only done one year, and that was straight Abeka w/ preschool worksheets for my son thrown in. I would always try to plan one week or more in advance, but that didn't always happen. What saved me was an outline. Have one for each day so you know what subjects need to be taught. Then, if you've been unable to prep beforehand, you just grab the outline for that day of the week and go from there. I imagine, copies could really kill that plan, but I didn't need to make any this past year. We're buying a small copier this summer.
Perhaps that will get you thinking in some way. Now I offer the disclaimer that, as I said, I've only done this one year. Next year I'm doing an assortment of stuff (not straight Abeka) and will be doing two grades. I'll probably be back here in three months begging for help, lol.