I am no cosplay expert. I've only done this twice! But I have done a few events in my time and, in a way, prepping for con is like a mini-event to me.
So like any trip or activity I do, I write pre-event and post-event notes. I feel a little silly when I'm doing it, but I always turn back to them and thank myself. Between anime conventions you may forget that you are low on hairspray or that a buckle came undone. Don't struggle to remember - write it down!
Record Your Supplies Before the Event
Lists should be your best friend in all aspects of your life. I list all the supplies I need to pull for each con and costume. No detail is too small. Write down every brush, piece of make-up, and back-up item. I actually put it all in a box a few days prior, but the list helps to gather my thoughts. Then, you can look back at your list after the event to note if you ran low on a supply or maybe you didn't even need it.
Google Docs Is Your Friend
Taking the lists a step further, I share my musings in Google Docs with my con friends to cross-check supplies. That way, you don't have 5 bottles of hairspray but forget the eyeshadow. The Google Doc is also easy to find the next time around so you don't have to recreate your lists each time. The magic of the internet means the list is always with you as well; no losing a piece of paper.
If you want to take your list planning a step further, include the food and hotel room supplies in that google doc. That way, you never leave your Moogle Hat at home by mistake.
Improve As You Go
You know you'll have some downtime at con, so write down your costume comfort level then. It's easiest to remember what works when you're still wearing it! Yes, you've done test runs, but never for six hours and now you're noticing that your skirt is slipping after two hours of wear. Make those notes for minor (or big) improvements along the way to avoid the issue next time.
You can even list those items in your google doc from your smart phone. Genius!
Read Your Notes
The most important step in this process is reading your notes. None of this matters if you don't use it, so be sure to follow up with yourself. I've had many "ah-ha" moments when reading my post-con notes. Now you look professional, prepared, and ahead of the game each year by simply writing a note.