Shake isn't the only awkward gesture. In general, multifinger gestures are trickier than their single-digit counterparts, and people have a corresponding reluctance to embrace them. Even the beloved pinch-to-zoom gesture makes for an unwieldy gesture, essentially impossible when holding the device in just one hand. For some with disabilities (or others juggling a latte), two-finger gestures are a challenge.
That doesn't mean you should avoid two-finger gestures entirely. With its ability to track several fingers at once, the iPhone's multitouch screen enables a range of innovative gestures. (The iPhone can support up to five simultaneous touches—for five-finger gestures—but the small screen crowds this full-fisted approach.) Don't be afraid to experiment, but consider offering single-digit alternatives to your finger-splaying custom gestures, particularly for common actions. The built-in Safari and Photos apps, for example, both let you double-tap to zoom in and out as an alternative to using the two-finger pinch gesture. Never underestimate the value of one-handed phone use both for convenience and accessibility.