Charlotte's post on characters got me thinking. I agree, it can be very difficult to keep track of characters, particularly if I'm reading the book very sporadically with gaps in between for other things like holidays, family visits, or editing. When reading a book, I sometimes have to flip back to see who the character was and what their purpose was in the story. This is harder to do with ebooks, where the "flipping back and forth" is much more cumbersome.
There are a number of tricks authors use to try to ensure readers remember the middling and minor characters. One is to give them names that are distinct from each other, or names that fit the character, either in ethnicity or age or even class and job. Thus Mario for the Italian plumber, Ethel for the old lady next door, Bob for the farmer down the lane. One can play against type in this name game too, and name the old lady Krystal and the farmer Raj. Either way, readers will take notice.
Another way to keep characters unique is to give them a vivid, distinctive trait of some kind. It can be appearance, such as a red beard, or a speech pattern, or manner of dress, etc. A brief description that creates an instant, vivid impression is better than a long description of height, hair, and clothing. I was recently watching a period TV show in which many of the men were in uniform and all had brown hair and moustaches. I couldn't tell any of them apart, so gave up on the show.
Choosing character names, both first and last, can be an enjoyable challenge. As noted above, the name should reflect the age of the character, and for this, checking popular baby names for the decade of your character's birth can help. In my age group, every second girl was a Barbara, but the name has barely been heard from since. Make sure the name matches the ethnicity and geography. Luckily the internet is an boundless resource. You can Google common Slovak names, both first and last, and also names that are common in a particular region through online phone listings. Elegant names like Nigel conjure up a different character than earthy names like Buck.
Last but not least, as Charlotte said, names mustn't be too similar. Five names that start with J or contain only one syllable, for example, will have readers saying "Who's Bill? Who's Bob?" One of my jobs during rewrites is to divide a page in half lengthwise for first and last names, write the alphabet down the left margin, and fill in all the character names in their proper box. That quickly shows where the problems lie and what letters are being underrepresented. Usually it's easier to change minor character names so they no longer conflict.
Which brings me to the final trick to making characters memorable. Don't have more of them than you absolutely need to tell the story. I'm not a fan of "how to" books that pretend to tell you how to write a great book, but I learned this tip in a workshop years ago and I still think of it today. Consider all the characters you've put in your book and the role they play. Then ask yourself whether you could eliminate a character by having another character take on two roles. If you do this a few times, you have reduced the number of characters while simultaneously making their relationships more complex and layered.
Happy writing!
