If you need to remind yourself to do something with a booking (eg. collect a deposit, collect a pre-order, chase a booking) use the "Follow Up" feature.
Located at the top of the booking details on the left hand side of the screen, click into the box and select the date you want to follow up with.
In the Enquiries page, the follow ups will appear as a clock symbol next to the enquiry. When you hover your mouse over the symbol, you'll see what date of the follow-up. Your most urgent follow ups, i.e. any for today's date or past dates, will show in the 'follow up required' section of your Notification Centre.
Don't forget: Once you have dealt with the follow up, open up the booking and delete the date so that it doesn't just sit in this high priority list. You can delete the follow-up by clicking on the cross and pressing save.