I've posted this before, so I just cut/pasted it...
Proprioceptive neuromuscular facilitation.
Sounds funky, but I'm sure you've seen it before. An example: Lie on your back. Extend one leg above you as far as you can stretch on your own. Have a partner hold your leg at that position, and push as hard as you can against them with that leg. Push for at least 6 seconds. After the 6 seconds, exhale and have your partner push your leg farther than it would stretch before. Tell them when you can't go any farther, and hold it for a good 15-20 seconds. You can do the same thing with different stretches. It has to do with the Golgi tendon organ, but you can google it for specifics.
For your kicks, I'd do the PNF stretching while standing, with your leg out to the side, and your partner can do the same thing from that position. Again, you will abduct (extend to the side) your leg with the assistance of a partner. When you can't go any farther, your partner will hold that position, while you try to force your leg back down for 6 seconds. Then your partner will lift your leg again as high as you can go. You can do some of these stretches by yourself, but most require a partner.
Oh, and make sure you always stretch after a workout, no matter what. I'd stress post-workout stretches more than pre-workout.
NOTE: Do some research on PNF stretches, and get a good idea of how it works first. For kicks, I'd suggest doing the leg stretches to the sides with your leg straight as well as with your leg bent at the knee, like your leg is cocked for a roundhouse. When doing any stretches to increase flexibility, go to the point just short of pain, take a deep breath in through the nose, and slowly exhale through your mouth while furthering your stretch. You'll go from inches short from reaching your toes, to touching your wrists to the floor in no time. Intense stretching should be done when your muscles are already warm, such as after a workout.