What you need is an on-line checklist. You're only human after all. At web-sites like promspot.com or promsplus.com, you can create a free profile and enter details about your prom night. The site will generate a checklist for you and send you email reminders about what you need to accomplish each week. This is a pretty fool-proof plan, because you can even print out the checklists and take them with you on your day-of-prom errands.
For extra insurance, find out if your dress shop, flower shop and hair salon will call to confirm pick-ups and appointments with you. Most places are happy to do this. Also, try to reserve some down-time for yourself on the day of prom. Give yourself a good two hours to take a bath with a good book or take a long, calming walk. This is your night. Don't get caught up in the stress and drama.