Seeing if your desire for the item lasts the test of time (weeks or a month) before buying it is a great strategy. I'm always amazed at how my desire for an item cools off during a waiting period.
And if you find your wardrobe-gap items on sale earlier than you expected, that seems like a good purchase.
An additional strategy that helps me avoid clothing purchases I'll feel guilty about:
I narrow my focus.
My wardrobe is made up of four neutral colors and four color-colors.
And I made a list of the lines, silhouettes, and details of the clothes that flatter me the most.
I do shop mostly at sales, but I stay within my focus.
I have fun shopping only sale and clearance racks. Every purchase has to fit within my wardrobe colors, and has to fall within the lines, silhouettes & details that flatter me.
Anything that doesn't fit those parameters (which is about 99% of what's on the rack), I know will be a wardrobe orphan and a regretted purchase. I've bought too many of those "regret" pieces in the past - so it's easy for me to leave them on the rack now.
I've made some wonderful finds on the clearance racks - it's a fun sort of shopping serendipity! But I don't always find things that I deem worthy of buying when I shop.
My wardrobe is smallish, but it's complete enough that I'm not in desperate need of anything - so I'm seeing how long I can go by rounding out my wardrobe with my sale / clearance finds.
My previous wardrobe was a mish-mash of all sorts of colors, few neutrals, and it was impossible to pull together a head-to-toe cohesive look.
My new wardrobe that focuses on specific neutrals, colors, and styles is actually freeing instead of limiting. I can really use each piece in a lot of different ways, so the limited color / style parameters never look (or feel) like I'm repeating things. And I can easily pull together outfits that work well for me.
I love this wardrobe versatility so much that it's easy to say "no" to the pieces that aren't a good match for my existing clothes.