It's not easy, but there are little tricks you can follow.

When choosing names for your characters, the first thing to pay attention to is the location where the novel takes place. Every state has a different surname that is popular. For instance: where I live in Vermont, I'm 30 miles from Montreal, the French quarter of Canada. Although I'm in VT, there are a lot of French first and last names in my town.
When I lived in a certain area in Florida, Johnson and Williamson were popular last names. In the area I visit in Long Island--Shirley--there a lot of Italian names. Paying attention to names where the novel takes place adds authenticity to your work.
If your novel is based in a town or state where you live, using the phone book to find authentic sounding names works as well. When using someone from say, South America, choose what area you're writing about and use Google to find names for that town or city. I used this method in the novel I'm currently working on.
To me, though, the most important thing to be wary of is initials. I've edited novels where characters' first names were for instance: Mary, Mark, Melanie, Marsha. This is soooo confusing. To avoid this, make up a list of characters' names as you go along and watch for those repetitious initials. I know it's hard. I inadvertently chose a lot of similar names in my first novel. I wasn't aware of it until the novel was finished and edited. Then I had to re-choose names, change them in the manuscript and hope I got every one.
Any questions? Either leave a message here or contact me at: danar96.starband.net.