For me, it is making use of dictionaries often. I'll look up the meaning of the word and then when I encounter it again and forget its meaning, I will refer to the dictionary again. I don't bother how many times I've to check, but I will re-check every time I forget the meaning of the word/s.
Whenever I encounter any word I'm not familiar with, I'll refer to a dictionary. I'm very persistent. I think there's no shortcut. Maybe you're not persistent enough. You're reluctant to refer to a dictionary. I even suggest you have a few dictionaries because sometimes, after referring to one dictionary, you may not understand clearly the meaning of the word. If such is the case, another dictionary may give you a clearer defiinition.
I hope what I say sound logical to you.
Only by being persistent do you remember the meanings of words which you find difficult in remembering.
I hope what I've said sounds logical and helpful to you.
All the best.