Apparently doing 'warmup' stretches doesn't make much difference. But I definitely stretch after, especially my back, quads,hamstrings and hip. Not stretching your hip will tighten the IT band and cause knee problems.
One stretch I also like is for the neck. Lock your fingers together with your palms out and stretch your arms over your head and back a little bit. THen tilt your head foreward, pressing your chin toward your chest. It feels great! I find it's really good after long rides or even when you stop during a ride.
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You can catch more flies with honey than with vinegar. That's great...if you want to attract vermin.