(From a non-Steve:) Google quickly comes up with the 'standard' stretch. Looks like http://www.footstore.com/wp-content/uploads/2009/12/hip_flexor_stretch.jpg
That's probably a good one too.
Not quite what I was saying. The hindmost leg in the picture would have it's shin up against a wall. The stretch in that picture is probably better for me than the one I try to do because I might be able to do that one properly as it looks like less of a stretch.
I hope I wasn't too dismissive of Steve's advice But it doesn't relate directly to muscle pain, at least not pain that is clearly DOMS-related. It's in the quads, not the hip-flexors.
I'm more on about my permanently being better at going up stairs than down. I'm sure that I'm faster going up stairs than I am at going downstairs. If my legs are shot after a hard ride then everything is harder, not just stairs. I never feel discomfort in my hip flexors, even after a hard ride. It's always my quads that hurt and sometimes I get grating, grinding achilles heels.
As for this spine-curving business: as-a-layman, I reckon this is more about improving your posture. But a stretching regime can't hurt
(This is all from personal experience - for all I know, someone with tight hip flexors may well be getting some new and interesting pain when walking down stairs!)
It's very much about posture. I've seen a number of aged mileaters that can't stand up straight because they have tight hip flexors. I've looked at my back in a mirror and made it straight and found that I tend to move my thighs towards my chest a little bit. This is because my tight hip flexors are pulling them that way. When I straighten my legs and stand upright, my hip flexors pull my back to an inwards curve which gives me my aerobelly unless I tense up, straighten my back and try to keep my legs in line with my body, then I can feel a slight stretch in my hip flexors.
I could get rid of my aerobelly by tilting my upper body forwards to re-straighten my back. Then I'd be leaning forwards all the time when standing up. I can keep my back straight easily if I'm sitting in a chair. But what I really should do to improve my posture, is to stretch my hip flexors so that I naturally have a straight back when I stand upright. It would also mean that when I go down stairs, I won't be stretching my right hip flexor to lower my left foot (or vice versa) onto the next step down, I'd just be able to do it without stretching and probably distorting my back. After a hard ride, I sometimes lean forwards by hinging at my hips and grabbing onto the hand rails, so that I don't go headfirst down the stairs, to make going down stairs easier
I think this is why a lot of fit cyclists who aren't fat have an aerobelly. We spend a lot of time in the crouch position developing our thighs, but neglecting to stretch our hip flexors, even if we may be doing hamstring and quad stretches.