Be sure to practice slow tight maneuvers more generally - find an empty car park to drill figures of eight, circling, and weaving too. Get as comfortable as you can with managing the balance of the bike at low speed.

Once you've established that the turn is safe and committed to it, keep your eye on where you want to go (edit: i.e. look forward in the new direction, not down), don't get distracted. This will help with balance.

Be sure to maintain tension on the rear wheel. This is a combination of slipping the clutch and dragging the rear brake (this works well for low speed maneuvering as well. Turn your head and look where you want to go. Looking directly in front of you may cause you to fall. You can also lean forward to help maintain stability.

If you ask me, the biggest contributor in failure to complete the U-turn (or figure 8, or S bend, or whatever low speed maneuver your testing authority makes you do) is not technical or mechanical, but rather anxiety. In hyperfixating on not hitting the boundaries, i.e. the sides of the road or the stripes, you often induce underconfidence in yourself which will cause you to hesitate and fail to commit to the maneuver. And the thing about low speed turns on two wheels is that you must commit to the maneuver, because if you slow or stop suddenly you will fall down, or at least go tipsy enough that you won't be able to resist putting a foot on the ground.

I am not going to offer any advice on technique because others have covered that already (drag the rear brake, etc.). Instead try this:

Go to a big parking lot with absolutely no obstacles or barriers or anything around you. Balance your GoPro or phone camera on something so you can record yourself. Ride out and back, and do the absolute tightest U-turn you can manage. Don't think. Don't worry about how big or small of a turn you're making. Don't look at the ground. Don't hesitate. Just do it.

Review your footage. Go back out and chalk where your tires went. Measure the distance. Compare it to a road. I'll bet you a penny or whatever your local currency is that without the added pressure of worrying about barriers, your turn was small enough to fit into two traffic lanes, exactly as required.

All that said, I don't understand the obsession testing authorities have with forcing people to do twiddly little U-turns and loops and circles at parade speed. None of that shit will successfully teach you how to ride a motorcycle down the road. It's pointless frippery for the sake of criticizing people over something that's truly meaningless in almost all situations. And the only reason they're so keen on it is because you can do it for free in a parking lot while the instructor stands there with this stupid little clipboard, and none of it requires anybody actually going out on a real road with you into the real world.

Once you get your license, if you have to get your bike out of a tight parking spot and that requires doing a three point turn and putting your foot down in the middle, is an instructor suddenly going to pop out of the bushes and dock you because your foot touched the ground, god forbid? Of course not. None of that bullshit is going to teach you how to avoid obstacles at road speed, or predict when a speeding car is going to cross your path, or what to do if you get into a tank-slapper, or how to react if you found you entered a corner too fast, or how to ride in the rain, or how to handle getting a sudden flat tire.

All of those are real world situations that cause people to get hurt on motorcycles. Slowly tipping over in a parking lot, conversely, is typically harmless except to your pride and possibly your chrome. If the choice is between waddling your bike a couple of feet or just showing off by doing something technical and risky but not putting a foot down, I'd say put your foot down every time. (And yes, before anyone comes at me I am perfectly capable of doing a tight U-turn without putting my feet down, or even burnout turns on gravel or dirt, weaving between the cones, balancing in the gap between the two yellow stripes, doing the 90 degree corner and stopping with my front tire in the box, and all the rest of it. In reality I have never actually had to use those skills at any time other than to pass my license test and for showing off. Literally never.)

I did my endorsement test on a little 1975 Honda cb125s that weighed 200lbs (90kg) and had a very short wheelbase and a top speed of about 50mph (80kph) before floating the valves in top gear.

Barring that kind of cheat code, I like to stay in gear and idle/use a small amount of throttle to maintain balance (this will help the bike try to stand up in the corner). You can drag the rear brake and/or slip the clutch if you need to control speed. If you can easily raise your idle, this can help if you don't have a steady throttle hand.

The most important part is to commit to the corner when you tip in.

Practice making continuous circles in an open lot and reducing the radius and speed. Get a feel for the bike in the middle of the turn, then make the path elliptical to get a feel for tipping in.

Whatever you do, don’t do the Tim Horton.