1) Practice splits everyday (this is a sure way to benefit from stretching)
2) If you eat a lot of meats/cheese/milk products, then lower your intake
3) Time in splits should be at least 2 minutes in one sitting. Multiple times are better and will get you down quicker.
4) As mentioned above, find a firm but soft object that is about 6 to 12 inches high that you can prop your foot on (a rolled up sleeping bag). Prop your front foot (heel only) on the object and gently push your back thigh down to the ground. These are oversplits and they are VERY helpful. You are using gravity to help. Try to make sure your back leg is straight and that your knee is correctly positioned.
Frequency and duration are key. The more frequently you practice, the quicker you will be down. When you can touch your thigh to the ground, you will notice that your regular split is painless and EASY.
Also, stretching your back, arms, shoulders is very important. Tims_Dad mentioned the stretching book....and I agree. So many gymnasts don't really see that if they improve their flexibility, then they will have dramatic gains in accuracy, balance and control!!!!!!