Some Preliminary Observations
These are not rules. These are just some observations I've made and conclusions I've come to. The names of your characters matter. With Historical Romance in particular, I think it's getting harder and harder to find good names. There are so many "Lord Something-ham's" out there that certain name forms are starting to sound trite and shallow, and once a name sounds trite and shallow that effect transfers to the character.
Try not to have characters whose names start with the same letter or sounds. Why? It can lead to confusion since readers do not have your investment in the story.
You need a name that sounds English, sounds cool and even reads cool. That is, you must consider how the name looks on the page, how it sounds in your head and how it sounds if said out loud. All three count.
A Rose is a Rose is a Rose, right?
Well, no. We all know names shouldn't matter, but they do. Your characters need to have names that suit them and your story. Some stuff is obvious; don't give your characters names that weren't used in your time period.
Other things are less obvious, such as even if you've confirmed a name was used in your time period, don't use it if it sounds modern. Maybe Tiffany was a name used in the Regency (I doubt it, but maybe). But even if so, Tiffany sounds modern. It's great if you can find a name that lends itself to a good nickname, too.
Broader considerations
Try not to have characters whose names start with the same letter or sounds. Why? It can lead to confusion since readers do not have your investment in the story. Particularly at the beginning, your readers have no idea who's who. They're going to have a hard time keeping track if you throw a lot of names at them, let alone ask them to remember that John is your hero, James is his best friend, and the butler is Jones.
A Few Things that Bug Me about Names in Historical Romance
One thing that really bugs me is the use of actual titles. There was a duke of Portland, people, so if you give your hero the title of the earl of Portland, it's going to seem like you didn't work very hard when naming your character. The point is that the names of your characters should not confuse your readers or give them pause. As in "The earl of Portland? Doesn't she know there's a Duke of Portland?" Clunk. Your reader is out of the story.
Names that are becoming trite: Common english suffixes such as:
- -ham
- -sham
- -land
- -ton
- St. [anything]
Successfully or not, I work really hard at naming my characters. Here's a page that shows when, how and why I changed names in The Spare. Check out the process I used to arrive at the hero's name and his title in my Work in Progress (The Heir).
Extra Credit
Make a list of the names of all your characters and try to give them all distinctive names.