Squeeze your bum - all the time - like you're trying to crush a grape.
Glute engagement is HARDLY EVER mentioned in TI. I seem to recall
it's mentioned in passing, just once, in one of the newer TI DVDs.
Very interestingly, if you follow the TI twitter account, the most recent post
gives a link to a video on Tim Ferriss' website, where a student was taught
TI by Terry, with Tim in attendance also. There is a point where the student is struggling with horizontal balance, and Tim gets her to do a sort of plank exercise, grabbing Tim's ankles on the pool deck, and raising her legs by engaging her glutes and core muscles.
16 minute mark for the reverse hyperextension exercise.
The glutes are a hip extensor, so they straighten the body and keep legs in line.
Glutes engagement also (in my experience) greatly helps to keep scissor kicks and wayward waggles in check.
It is a miracle cure. Please try. It WILL take several weeks or months before continuous glute engagement becomes automatic, because the brain has to develop new neural pathways.
Glutes engaged, but knees not locked out rigid.