Story Editor's last tip is important: you want to feel that click, because finding the right name can really help you get in the groove with your character -- and later for your audience to do the same.
Working on a new script with my co-writer, we started out naming our characters after the real people they were based on. This helped in the beginning because it gave us a clear sense of the type of person we were dealing with.
But a word of warning about naming your characters after friends and enemies: at some point, you'll need your character to diverge from the real-life person -- you want them working for the script, not being faithful to someone your audience doesn't know or care about. You may want to rename them at that point to give yourself mental freedom from the person who gave you the original inspiration.
Say the name out loud a few times, and don't forget to combine it with any titles your character might have to make sure it works in the ways it'll be spoken by other characters in the script.
Look at your cast names as a set and check them against the rules above. For extensive casts, I've started keeping an A-Z list on file for quick reference. I also keep a file to save names I haven't used yet but that I think might be useful in future. Here are a few of my go-to online naming resources:
Facebook.com - Go to the page of the biggest social butterfly you know and cruise through their extensive friend list as a quick way to find first and last names.
YeahBaby.com - This babynamer allows you to search by beginning and end letter as well as number of syllables and ethnic origin.
Popular Baby Names - The American Social Security site provides lists of most popular names going back for a hundred years. I find my character's birth year by adding a couple of years to today's date to adjust (optimistically) for development and production time and then subtract their age. Search the SSA site by year to find naming trends appropriate to your character's age.
Behind the Name - Surnames by ethnic origin and/or first letter. And for some of these, you may want to refer to this handy dictionary of surname pronunciation. Googling your setting name and "surnames" will also provide you with genealogy sites listing common names from where your characters live.
Imdb - Use the drop-down menu search at the top of the page to select "Characters" and enter the names you're thinking of using to see where they've already shown up in film and TV. If the name you want has been used for an iconic character already, or is in circulation on a popular current show, you might want to avoid the association.