After passing, how to keep bad habits away?

I started driving a few days ago after passing around a month or so. I decided not to use "P" plates to keep lunatics away. But, now that I'm out in the wild, not just driving within 20 minutes away from the test centre, I can spot that far more people are not driving "by the book" if that makes sense.

I was staying within the speed limit everywhere I went, but felt a lot of pressure a couple of times to go faster, especially when a few folks were tailgating on national speed limit roads. Same with overtaking/undertaking, trying to drive properly (not undertaking traffic on the right, allowing space between more vehicles) I only see some folks annoyed or getting competitive.

Now, I'm not just ranting, I am genuinely concerned that lack of observation (no instructor examiner in the car) and other bad lemons can influence me into building bad habits in the long run. All these peeps have passed their tests, so they were up to the driving test standard, but now some of them are driving nuts.

How do you keep yourself sharp? What do you do to continue to improve without regressing? Share your tips and tricks!