i've never been able to touch my toes without bending my knees. from the research that i've done, i believe that improved flexibility in the hamstrings and lower back is the best way to reach this goal. I was wondering what the key stretches are in order to promote this. i've been having trouble finding some goods one.
The best stretch I've found for the posterior chain (touching your toes) is a dynamic stretch called the flexibility rollover or half reverse roll. It's fun and really gets you deep into the stretch. You'll be able to touch your toes in no time. Here's a video of the stretch in action (another video here).
Honestly, the best training for touching your toes is just doing it. Stand with your legs straight (although not with your knees bent inwards) and lower yourself as far as you can. Then, stay in that position for a few seconds, try to relax and stretch just a little bit further. When coming back up, again, do it slowly (the general advice people call out is to "feel every vertebra go back in place") and return to standing. Repeat whenever you have a spare moment. If you find that your back hurts from it, try adding a slight backwards bend after each stretch to stretch your back out in the other direction.
As with most stretches, it takes time and patient effort. There are all sorts of theories about "bouncing" to get a further stretch (generally not recommended) and "dynamic" versus "static" stretching (the latter is what most people think of for a stretch, just moving to that furthest extension and holding for a few seconds. The former involves going to about 90% and then flexing the muscles involved while holding it), but the prime part is to just do it day after day.
I had a huge growth spurt in high school and my muscles never lengthened out. Trying to touch my toes with straight knees and back led to excruciating pain around the back of the knee and upper calves. The way I got through my stiffness problems was actually with several support stretches. Once I had loosened the muscles connected to my hamstrings, I got much more out of straight legged stretches.
Here are the ones I used:
Once I started practicing these, I could much more easily get into the proper position for the standard toe-touch stretch and didn't experience as much pain around my knees. Hope this helps!