' Yoga texts recommend vaseth, which means one should stay in this posture for for a long time. Even a stay of five minutes has a tonic effect on the posterior muscles, the abdominal muscles and the pelvic organs, because of the rectal and abdominal locks'.
Srivatsa Ramaswami Complete Book of Vinyasa Yoga p 75
Paschimatanasana is one of only two asana that I remember Ramaswami giving hands on adjustment, he would push forward our sacrum as we folded forward into the posture (the other adjustment was in tadasana where he would grip the pelvis and lift and hold up while we would do the hand arm variations).
In dandasana (pic 1) stretch fully as in standing tadasana, lifting up out of the pelvis, fold forward from the pelvis, imagine pushing your backside back as when we practiced uttanasana (standing forward bend).
The slight bend you may find in the knees is GOOD, this is protecting your hamstrings, eventually, over time your legs will be able to stay flat.
It took me about a year before my legs would stay flat on the mat, there was always a slight bend of my knees.
Forcing the legs to flatten could damage your hamstrings, this is not fun and even a tweak of the hamstrings will take weeks if not months to pass completely. It's the pelvis we're most interested in working anyway in this posture, be kind to your hamstrings.
As you become more comfortable in the posture, a gentle way to work towards getting your legs a little flatter ( not necessarily all the way) is to spend a minute or so in the posture then shuffle your sit bones backwards this will gently lengthen your hamstrings and allow them to flat them a little more. Settle for another minute and then shuffle back a little further.
Engaging mula bandha ( drawing up the anus ) helps to protect your hamstrings, it seems to activate the gluteus maximus taking some of the stress off the hamstrings.
Engaging uddiyana bandha, sucking in the belly creates space for you to deepen your forward bend.
In Vinyasa Krama, in an extended stay in paschimattanasana we may lengthen the exhale and even hold the breath at the end of the exhalation and engage the bandhas strongly.
Inhalation 3-5 seconds, exhalation 5-10 or even 15 seconds.
In vinyasa Krama the chin is tilted to the chest and in an extended stay we may engage jalandhara bandha, the throat lock more strongly.
Occasionally I find tatkamudra a useful pre paschimottanasana posture, here's a link to a post concerning this http://tinyurl.com/6egyxoy Here's the gist of the post.
The other week, while practicing Primary series I was trying to settle into paschimottanasana but was feeling a little stiff. I laid back on the mat for a moment and figured while I was there I'd get my bandhas warmed up, better to engage them in the forward bend ( I tend to spend five to ten minutes in paschimottanasana, Vinyasa Krama style). So I raised my arms over my head for tatakamudra stretched and at the end of my exhale stopped the breath and drew up and back mula bandha, connected it to uddiyana, drawing my abdominal muscles inward and backward and bringing the small of my back onto the mat. A few long slow breaths and I went back to paschimottanasana, low and behold, the stiffness was gone and paschi felt comfortable enough for a long deep stay.