if you have a smartphone, you can use the animal crossing new leaf guide app to keep track of the items you have in your catalog! it's also useful for keeping track of birthdays and how many creatures are in your encyclopedia!
I'd also suggest using the guide app or some other means of keeping track. Also remember to check what items villagers are posting to you, I completely forgot and ended up having to go back and check my whole catalogue again in case they'd given me something I hadn't got.
I check every shop everyday apart from the museum, some days I'll just tt several in game days at once just for the sake of getting more items collected. But when you get to the point of near completion you will probably be best using a catalogue town rather than hunting around for ages for a couple of things. Good luck!
I bought the prima game guide in 2014, which is a little outdated now (it doesn't have all the DLC or Streetpass items) but it has checklists with pictures of every item in game and where/when to get it, museum donations, how to complete special events, etc, it's a completionists dream
I also have a spreadsheet and a mobile app as backups.
Cataloging parties on other websites. I know this might be home to some but your better off looking for sites that while may not be as big as the forums here have more chances to do this.
Also all of the above mentioned.
Get a checklist started so you don't buy the same thing again.
I used moriDB checklist and made a lot of Catalog for each type. Example Shoe/stockings.
Just put what was missing in each Catalog so it was easy to find.
Look online to see if anyone is hosting cataloging parties. For the clothing Catalog Party I wore a wetsuit so I didn't try the clothing on by mistake. It goes very fast. Someone hosted all the furniture in sets before they started their town.
There are service sections on some boards where you can pay bells for hard to get items like the sloppy sets and Japanese items.