Flexibility is always great to shoot for, but it's a pretty broad goal. Do you want to do splits? kick high? yoga? Narrow down your flexibility goals and you'll have a lot more success.
In general you want to be warmed up before doing any kind of stretching, a good analogy for your muscles is taffy. Warm taffy stretches, cold taffy snaps (ouch). Do 2-5 mins of activity that gets your heart pumping then get on to the stretching.
If you want dynamic flexibility, aka kicking high and such, a good course of
dynamic stretches will get you there. (These are also the ideal kinds of stretches to perform before a workout as well.)
Example dynamic stretches: To minimize injury remember: You are not bouncing. Your limbs are never in freefall. You maintain control of your limbs at all times.- Forward kick: Hold out your hand in front of you. Kick your straightened leg forward, toward your hand in a smooth, controlled motion until you feel a slight stretch, then bring it down. Repeat for 2 sets of 14, each leg.
- Side kick: Hold out your hand out to your side. Kick your straightened leg sideways and up, to your hand in a smooth, controlled motion until you feel a slight stretch, then bring it down. Repeat for 2 sets of 14, each leg.
- Back kick: Bend forward and lean on the back of a chair. Keep one foot planted on the ground, and kick the other straightened leg backwards in a smooth, controlled motion until you feel a slight stretch, then bring it down. Repeat for 2 sets of 14, each leg.
- Arm swing: Stand with arms forward, palms together. Swing your arms behind your in a smooth, controlled motion until you feel a slight stretch at the back, then bring them forward. Repeat for 2 sets of 8.
- Arm rotation: Stand straight. Pull your shoulders back and stick out your chest. Keep your elbows straight and rotate your arms at the shoulder in a smooth, controlled motion to your full (comfortable) range of motion. Repeat for 2 sets of 8.
- Back bend: This is like the yoga "cobra" pose. Lie face down on your stomach, plant your palms next to your shoulders like you're doing a push-up, and push upwards, but instead of doing a push-up, press your pelvis into the floor so that you're bending your back. Go to your full (comfortable) range of motion.Repeat for 2 sets of 8.
I'm repeating myself, but I cannot emphasize this enough, maintain a smooth controlled motion for all of these exercises or you will pull something.
Static passive stretches build up static flexibility, and these are some of the stretches you can do to prepare for the splits. They are also best performed AFTER your workout during cooldown. These are the classic stretches where you sit down, stick out your leg, and lean forward or whatever. Basic stuff, stretch and hold for 8 seconds or more, repeat. There are plenty of these online for every muscle group imaginable.
Static active stretches can be difficult. During these stretches you move your body into position and use your muscles to maintain that position. If you're training for the splits, perform these stretches AFTER your static passive stretches. A lot of muscles are involved with doing the splits, and you gotta stretch each one of them, in the proper sequence (details in Kurz's book), before you do "the big stretch" that stretches them all. Examples are the side split stretch, where you stand on your feet and spread them outwards until you feel a stretch, maintaining balance. Another one is the forward split stretch, where you place one leg forward and one leg backward, then balance yourself on your feet.
I picked up most of this stuff from Stretching Scientifically: A Guide to Flexibility Training by Thomas Kurz.