First of all and probably the least effective is leather conditioner. Leather balms supposedly soften even the hardest of leather and ensure that the material stays soft, so conditioning your boots with a product like the Docs Wonder Balsam shoe protector can help. Apparently well-treated leather will be easier to break in, however, I wouldn’t put all your eggs in this basket.
Blister plasters can also be also surprisingly effective, if you put them on pre-blister. Those fancy hydrocolloid gel ones act as a great cushion between your heel and the shoe so you don’t get as many blisters. However, saying that, in my experience you put them on, and a blister just pops up in another place.
Socks are quite literally your best friend. A thick pair of socks or even multiple pairs of crew-length socks will do the trick for stretching out the shoe while shielding your feet, especially around the ankle area. Whilst trekking around with fuzzy pink bed socks under your Docs isn’t exactly ideal, this will help in the leather-softening process in the beginning. Your thickest socks will do the trick just fine, but Dr. Marten and Amazon have special socks that have extra padding on the heel, ankle, and arch if your thick socks aren’t doing the trick.
Something as simple as a hairdryer can also work. Running heat over - but not too hot or close to the shoe as you’ll burn the leather - will soften them, making it easier to stretch them out.
Finally, my personal favourite, the silicone heel protectors. You wear them under socks and they go around your ankle and over the ball and arch of your foot. They serve as a better version of blister plasters almost because it covers the entire back of your foot and ankle. My only criticism would be you have to get the positioning just right or they’ll slip when you’re walking and you'll end up with blisters.
The most important advice would be to take your time, wear them around the house, or for a short walk and build up your wear-time. Don’t plan a trip away or a whole day out and commit to wearing a brand-new pair of docs, I can almost guarantee it will end badly. Every pair of Dr Martens are different, and some are significantly worse than others (shout out to the Jadon platforms, I don’t think I will ever break them in). Try everything and don’t give up straight away.