Try to be at the theme park when it opens; you'll have your best chance of riding the Forbidden Journey attraction with no wait. (The two main ways for guests to fast-track line-ups at Universal Orlando aren't available for Forbidden Journey, at time of writing -- see more about bypassing line-ups on the next page.)
Another reason to arrive early during high attendance periods is that the entrances to the Wizarding World may actually be closed during peak hours later in the day.
And finally, arriving early is in general a good strategy for enjoying any theme park: it ain't easy to get a family up and out the door and through the entrance gates by 9 in the morning, so if you manage do manage this feat, you can enjoy an hour of two of uncrowded time.
To get the earliest start possible, consider staying in one of the excellent Universal Orlando resorts. On many days, guests at these resorts get early admittance into Islands of Adventure. When we visited in late August, this meant that guests could enter Islands of Adventure at 8 a.m.; the Wizarding World of Harry Potter didn't allow entrance until 9 a.m. but still, the earlybirds who entered the park at 8 were able to enjoy other areas of Islands of Adventure and then were well positioned to be the first into the Wizarding World.
Tip # 2: Bypass the Line-Ups...