There are several simple things you can do now, at the very outset of your grant project, that will make life much easier later on:
Re-read your proposal and budget. By now it has probably been several months since you wrote the proposal, and no wonder if your recollection of the details is a little fuzzy. So do yourself a favor and take the time to carefully re-read your proposal and review the budget (both grant funds and any cost share). If you have co-PIs or collaborators, schedule a meeting together to review what exactly you proposed to do and how you proposed to spend the grant money.
Set up a timeline and workplan. After reviewing your proposal and budget, make specific plans for carrying out the grant activities. If you have collaborators, this is especially important. What did you promise to do? Who is responsible for each item? When and how will it get done? What outcomes are you going to measure, and how will you collect and analyze data to evaluate your success? What needs to happen now to get the project up and running?
Put reminders on your calendar for reports due. Remember how many times you've scolded students about not waiting until the night before the due date to start an assignment? Take your own advice. Ensure you aren't frantically scrambling to gather information for a due (or overdue!) report, by setting up reminders on your calendar well in advance of each reporting deadline.
Plan now for what you will report. Find out if you can get a copy of the funder's report format, so you know exactly what information you will be expected to provide in interim and final reports. As you set up your workplan, be intentional about building in documentation and evaluation along the way—so that, when it comes time, filling out the reports will be as easy and stress-free as possible.