Sit down with your legs as wide as possible against a wall or something and then keep pulling yourself in closer
Sit in as big a straddle as you can get and try and put your belly on the floor. keep moving your feet out wider until you are in a straight line
stand in a star shape with your hands on the floor...slide your feet down in a straight line as far as they go and put your elbows on the floor then hold it
Lay on your back with your legs in a straddle in teh air...using your hands gently push them down to the floor
If you're having trouble with straddle splits, you're probably a little bit tight in your hips. Here are a few things that help me loosen up my hips.
Yoga life magazine website: http://www.iyogalife.com/workouts/Flex_Time_Hips.shtml
This is an article on how to loosen up different parts of your body. (for other areas besides hips, scroll to the bottom of the page and click on 'related articles' any that say 'flex time' are from the same article) Hold the poses for as long as you can comfortably. I usually go for 2 to 5 minutes for the hip openers.
Another things that's helped me alot is a leg stretcher. http://www.amazon.com/Tiger-Claw-Steel-Leg-Stretcher/dp/B000LXAMXQ/...
I use this when I'm watching TV, just sitting in the postition for about 5 minutes two or three times a day. When the stretch feels too comfortable, I push my legs slightly further apart so I get a more intense stretch. Just be careful not to push your legs too far too fast or you could hurt yourself.
Remember, if you've been working your muscles to the point of fatigue, you can loose some of your flexibility. So if your muscles are over worked let them rest because chances are you won't be able to get into as deep of a stretch as you normally would, let alone be able to stretch further. Hope this helps.