Aaah good old flexibility. I'm a dancer (or try to be!) so flexibilty is essential. I'm not going to lie to you here, it's very painful to become truly flexible. My tip is practise, practise and more practise.
Like SL said, warm up a bit before you begin stretching and set yourself a little rountine you do every day. Put some upbeat music on to take your mind of the pain.
This is my little rountine I do each night:
Sitting down, legs in front. Reach past toes 16 times. Reach up to the ceiling, breathe in and then fold over legs. Repeat twice more but at the end stay down there.
Legs wide. Lean over right leg for eight, lean over left leg for eight, head on floor in the centre for eight. Try to get stomach flat on floor.
Lie on back. Eight kicks with the right leg, eight kicks with the left kick. Make sure other leg stays flat on the floor as it will be of no benefit. Grab right leg and pull until it's on the floor behind you, same with left leg. (The more you do it, the further you can get)
Do splits on right leg, splits on left leg, straddle.
Shake legs out.
Lie on back, knees bent in. Sit half way up and pulse your stomach for the entire song. (Hurts like nothing else!!) Whilst pulsing, go through the five Ballet arm postions. (Heh, working arms as well as stomach). Then pick your legs up and move them forwards and backwards whilst still pulsing and using arms.
Rest.
Roll over onto stomach and lift back slowly. Do it four more times and on the last one, arch right back and touch toes to head.
Stretch out.
Cool down.
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Obviously, you'll have to alter this for yourself but it just gives you a few ideas. If you want to get really flexible, you have to keep it up as flexibilty is one of these things you can gain or lose.