just keep holding them! hold it as far down as you can for maybe 30-60 secs, then push yourself a little further and hold. then get out of the split and rest for a min so you dont pull a mucsel, then do it again! after holding it a few times it will be closer if not on the floor.
Buy (and read): Stretching Scientifically: A Guide to Flexibility Training, by Tom Kurz.
It's the best book on stretching and flexability I've ever read. It's not too long, or complicated. It'll take about a day or two to read and absorb the information.
When I was about 28 (back in the olden days), I was actively doing Tae Kwon Do. I followed the methods and advise in this publication for a couple of months, and it allowed me to achieve a full front split, and full side straddle - to the floor, or and even elevated at one point. It also taught me the importance of 'hinging' at the hip joint rather then arching my back or forcing an unnatural position. All good stuff.
If you're mostly there already - this book will get you home.
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!!!!!!