Handwritten kanji are, by far, the most difficult type of Japanese text to read. Kanji are the most complex form of text (i.e. they've got the most strokes per character), so any quirks of a person's handwriting get multiplied that many times. The even bigger problem is that no two people's handwriting is exactly the same, so essentially you're always seeing each handwritten kanji for the first time.
I don't have any magic method to improve handwritten kanji reading skills, other than to simply practice/study as much as you can with kanji, and Japanese, in general. Being able to read handwritten kanji is often a result of your brain knowing what any particular kanji both could or couldn't be, and then your brain knowing what make sense.
As a simplified example, if I was looking at a handwritten sentence with one kanji I couldn't read (represented by ◎) that looked like this:
I'd probably think "Hmm...焼◎ has to be some kind of noun, since it's followed by を. The person is hungry, so they probably want to eat something filling, so what's a filling food that starts with 焼? Oh, yakiniku/焼肉. OK, so does the kanji I can't read look like maybe it's 肉? Yeah, it kind of does. OK, then ◎ must be 肉."
That probably sounds incredibly complicated, but the more experience/practice/study you get with kanji, and Japanese in general, the quicker the process becomes, and eventually it can become pretty much automatic, as your brain learns what is and isn't possible based on existing vocabulary, grammar rules, and context.
As another example, my wife (native Japanese speaker) and me (native English speaker) both have pretty similar eyesight. However, if we're walking around Tokyo, she can read street and shop signs (that are in Japanese from much farther away then I can. When we go to Los Angeles, though, the phenomenon flips, and I can read English sings from farther away than she can. There's no real difference in our eyesight, though. It's just a result of our respective language skill levels being able to decipher what hard-to-see/decipher text is by non-visual methods.