You can progress up to an L-sit by keeping your knees bent at first, and gradually straightening them as you get stronger/more flexible. So in essence you start out holding a pose like you are sitting in a chair, and work up to the L. (Heh, Corey is a fast typer.
I want to add, be sure you are working your L-sit and V-sit progressions while working on the ham flexibility. Don't wait for the flexibility before you start training the L. Training the L will also help develop flexibility!)
For more flexible hamstrings, here is a good progression from limited/beginner flexibility to very flexible:
1. Lie on your back with your legs straight out in front, spine neutral, abs engaged to stabilize the torso. Hug your right knee into the chest, letting the lower back lengthen. Switch legs.
2. As that gets more comfortable, gradually straighten the bent leg. You may have to hold on to the back of your thigh as you do so. As you gain more flexibility and control, you will need to use a yoga strap or a belt to hook around your arch/heel to hold your leg straighter and straighter.
3. When the supine stretches are comfortable, progress to seated stretches. Sit in an L-sit position on the ground with a very tall back. Bend one leg and let it open naturally to the side. Fold forward over the straight leg.
Be sure to bend at the hip joint, in the groin, NOT at the waist. Bending at the waist will stretch your lower back but will do very little for your hamstrings.. Switch legs.
4. When the one-legged forward bends are comfortable, start on two-legged forward bends. Keep both legs straight out in front of you and fold at the hip joint/groin as before. Try to flatten the abdomen onto the thighs and reach your spine long towards your feet.
You can intersperse the one-legged and two-legged seated stretches, no need to wait for the one-legged ones to be "perfect" before progressing.
5. When the seated stretches are coming along, add some standing stretches. Begin with your legs about hip width apart and fold forward at the hip joint just like you did for the seated ones. Gravity will help you here; it's okay to keep the legs slightly bent at first. Again, keep the spine long and be sure to not bend at the waist.
You can increase the stretch of #5 as you get more comfortable by gradually straightening the legs and also by bringing your legs closer and closer together. Be sure the legs are absolutely parallel when you do these.
6. Get to a plank position. Imagine there is a rope around your hip bones (about where you would wear a seatbelt--across the pelvis not across your waist). This rope slowly pulls up on your hips, causing you to have to walk your hands back towards your feet, eventually putting your body into an "A" shape. Basically you are walking the hands into Downward Dog Pose in yoga. Here is an image of what it looks like:
Downward Dog. Keep walking the hands out to plank and then back to Downward Dog, to get a dynamic stretch. Form is everything on this one.
7. Squat down with your legs together and hook your arms under your thighs, hugging your knees to your chest. Keep your chest glued to your thighs as you stand up, aiming your tailbone at the ceiling. Let the upper body "cascade" down the legs, keeping your chest as close to the thighs as possible, and the spine long.
You can also do walking high kicks for a dynamic stretch of the hamstrings. Requires some coordination and good stability though.
As always, be sure you are quite warm before doing any of these stretches. Best to do them at least after a full warm-up if not a full workout.
Hope that helps!
