How do you personally best remember the differences between any set of broadly similar things (colleagues/friends etc, different neighbourhood dogs/cats, different musicians etc). Whatever method works best for you can be adapted to work for birds. It took me ages to get my head around how to tell pipits apart, or the differences between Bar Tailed and Black Tailed Godwits, and which was which, and I'm still trying to get to grips with some of the larger gulls.
Practice works quite well, regardless - the more you see/hear the birds you're trying to memorise, the better. One day it all just falls into place - or you find a way that helps you make the mental connections you need. Watch videos on YouTube - they can add to your mental image of the bird in question (though not all of them are 100% accurate - possibly a good idea to check comments for mention of accuracy). Flick through a bird book and try to name the bird without looking at the caption first - the more times you see the picture, the more it sticks.