You should be OKAY on a Sunday in Mid-August. You don't have to worry about the church groups (it's Sunday) and the school groups. I go to Magic Mountain a lot and have a pretty good system.
1. If you don't have a season pass, purchase your tickets on-line before hand.
2. Get to the opening gates right when the park opens at 10am.
3. When you enter the park, avoid the temptation to head right for X or Tatsu. Break a RIGHT towards Goliath after you make your way down the "Main Street" area and hit the big Six Flags fountain. Goliath is a good first coaster and will get your blood boiling for the rest of the day. By nature, the park is designed to funnel people to the LEFT after the main entrance and walkway. If you start with Goliath, you can make your way backwards through the park and hit Batman, Colossus, Riddler, Scream and maybe Deja Vu within the first two hours of opening. I usually save X (and now Tatsu) for the last rides of the day. By that time, you're worn out anyhow and don't mind the wait in line.
4. Six Flags reaches it's crowd peak between 1 - 2pm. This is when lines will be the longest and the heat the worst. It's a great time to leave the park, head down the hill and have a leisurly lunch at either Red Lobster or El Torito.
Hope this helps! I'll be there on Sunday as well!!