As an occasional tester, in the couple of weeks before an event I’ll do a few extra aerobic rides (pettit, taku etc) and try and spend longer and longer in the aero position at different cadences. If I can do ten minutes on the turbo, the 25 or so minutes on the road is usually doable at more power.
My top tip: If you want to get good at TTs move to the UK, you can do three or four time trials every week!