Ten years in, there are katakana I still can't sight read: シ and ツ. I have a little mnemonic plus muscle memory workaround. It also involves ンand ソ because when I started I couldn't tell those apart either, until I noticed that ソ looks like its hiragana counterpart, そ. (In handwriting it is more apparent). So the mnemonic is "sheets and sew," (shi, tsu, n, so) and I spell this with my thumb: シツンソ, starting with an up stroke and alternating. Then I compare how vertical they are to what I am trying to read.
Faced with the prospect of having to do this in a grocery store, I have come up with a new trick. シ kind of looks like an s, and ツdoesn't. Not that there aren't kana which look way more like an S.