Los Angeles Airport (LAX) and Ontario Airport (ONT) are reporting that most flights will be "full or nearly full" this summer travel season:

Los Angeles Airport Authority Press Release excerpt:
Flights from Los Angeles International (LAX) and Ontario International (ONT) Airports this summer will be full or nearly full due to growing demand for travel and a tight supply of seats. Both airports are expecting another busy summer travel season this year.

If you are planning a trip this summer and don't have your airline tickets you'll want to start pricing dates ASAP. Tools like Kayak.com and Orbitz.com allow you to provide a range of dates that you can travel. Using a range, rather than specific dates, often opens up cheaper flights.

Here's a bit more about Kayak.com