Well, they had seat belts and various forms of harnesses to keep them in the cockpit. Although it was still risky, the belts were rudimentary and not designed with the best ergonomics. It was only by the mid-war that more purpose design seat belts like the Sutton Harness, which covered the shoulders, became regular equipment. Some aircraft still operated with a degree of risk, like the later models of the FE2 in which the front gunner had to unbuckle his lap-belt to use the rear gun. In this case, coordination between pilot and gunner was necessary to keep the gunner from falling out the aircraft. WWI flying was dirty and very dangerous work, and seat belts only eliminated one risk.