+1 for Molgrips, once into the massive pain stage only rest helps. I used to suffer from this a lot and have found the following have helped prevent reoccurence:
try to keep mobile if you can (walk around at work every hour or so if you have a seated job, go to talk to colleagues instead of email/phone, volunteer for non-seated tasks)
running. sounds counter-intuitive but apparently one of the few exercises that gently moves the sacro joint and associated muscles.
Do this stretch! I do it every day. Mr LadyAlex thinks I'm bonkers![url=http://www.google.co.uk/imgres?imgurl=http://images.teamsugar.com/files/users/1/12981/29_2007/pigeon.jpg&imgrefurl=http://www.fitsugar.com/Stretch-Those-Tight-Hips-Lower-Back-409369&usg=__fmMXZ94DZ2W1FwMb58rmzwA4aj8=&h=390&w=288&sz=31&hl=en&start=0&sig2=mpK1TwtoOZD_7QfkgO8n8A&zoom=1&tbnid=3443YNUgZMdYCM:&tbnh=116&tbnw=86&ei=F_cJTqSRHIy38QPE-dl-&prev=/search%3Fq%3Dyoga%2Blower%2Bback%2Bstretch%2Bpigeon%26hl%3Den%26biw%3D1920%26bih%3D890%26tbm%3Disch&itbs=1&iact=rc&dur=448&page=1&ndsp=70&ved=1t:429,r:14,s:0&tx=59&ty=99]
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