Keep it a one joint stretch. Many people want to jump right to performing a hip flexor stretch while flexing the knee. This incorporates the rectus and the psoas, but I find far too many people can not appropriately perform this stretch. They will compensate, usually by stretching their anterior capsule too much or hyperextending their lumbar spine.
You can strain or tear one or more of your hip flexors when you make sudden movements such as changing directions while running or kicking. Sports and athletic activities where this is likely to occur include running, football, soccer, martial arts, dancing, and hockey. In everyday life, you can strain a hip flexor when you slip and fall, for example.