If you don't feel like pressing the Home button, you can call up Apple's digital assistant app with a "hey Siri" voice command in iOS 9 — as long as your iPhone is charging or you're using an iPhone 6s or iPhone 6s Plus. Those limitations are designed to prevent battery drain, but you can actually access the shortcut anytime, anywhere on any iPhone just by keeping Siri active.
First, you need to activate the "hey Siri" voice shortcut via General and Siri in the Settings app. Second, you need to run Siri once and not say anything (or run a query of your choice). With Siri on the screen, the "hey Siri" voice command works until the display automatically locks, a time period you can set by tapping General then Auto-Lock in Settings.
Obviously the main downside is your battery is going to drain pretty rapidly, but on those occasions when you need hands-free Siri access for something — in the car or when cooking, for example — it's a neat trick you can use to avoid having to physically tap your phone at all. With new iPhones supporting "hey Siri" whether or not they're on main power, it looks like we won't need this workaround for much longer.