If you're a die-hard foodie, then your whole Los Angeles trip may be based around some specific restaurants you have in mind, and for those, you probably want to make reservations before you leave home. Opentable.com is a good resource for researching restaurants and making reservations. For the rest of you, where you eat will be more a function of what attractions you're visiting and where you are staying.
In addition to OpenTable, Yelp.com, and its mobile app are my best friends when looking for someplace nearby that has been well-reviewed by regular people. As a bonus, the mobile app tells me what's nearby that is open right now.
Here are some links to other resources for researching the trendiest restaurants:
LA Weekly Restaurant Section
OC Weekly Restaurant Section
LA Times Food Section
I don't review too many restaurants myself because I'm a picky eater (I confess all my eating quirks here), but here is the link for my Restaurant Reviews on Los Angeles Travel.
For special occasions, I've put together a list of the Most Romantic Los Angeles Restaurants with a View.
If you want to eat like a local, you might want to be on the lookout for some of the Top Gourmet Food Trucks tweeting their next location around Los Angeles.
If you want to take your Los Angeles dining experience to another level, you can sign up to take a cooking class from a local chef.