I find it much easier to do drag races if I set my phone on my desk or a table whenever I do drag races. This way you can focus on tapping 'launch' at the right timing as well as shifting more accurately.

Pull up a chair and lay your phone flat on the table (screen face up obviously), and then start the race. This helps with stability. You are not using one hand to hold your phone while using your other hand to to try to tap launch or to shift. This works great especially if you do not have a steady hand. The phone will be more stable. Want to have more control? Get yourself a stylus to tap launch or shift.

Turning is not necessary in drag races so having your phone on a flat surface is only recommended for drag races.