Never stretch before you warm up

The only reason it feels good to stretch even when you're cold is because your body sends out natural painkillers. To protect your tissues, you need to raise your cardiac pulse in order to fill them with blood.
The only way to do this is by working out.
Swimming is the safest way to raise your cardiac pulse. Water reduces shocks to the body because it reduces the felt effect of gravity on your body.
Jumping rope can also be effective, but it may damage your tibia's periosteums, which is the membrane enveloping your bones (except at junctions) that allows the blood to flow from your heart to your muscles connected by the tendon.
Swimming pools are sometimes inaccessible, so an efficient and safe way to raise cardiac pulse would be by cycling (except if you have knee problems).