i had to chime in on this one. like chris, im fighting a back injury due to tight hamstrings/no lordotic arch.
duncan - unless the mecahnics are there, you really shouldnt be doing any movement loaded. it sounds like youre pretty tight, so im guessing some of the form is compromised in ol, squats, etc., in which case flexibility needs to be the priority. the only progress ive made with my hamstrings is frequent stretching throughout the day. getting up at least once every 2 hours for 5 minutes or so. i had been relatively pain free with this protocol for a month. my back recently flared up again since being "too busy" at work these past couple days to get up. if you need some stretching ideas, i second robb's recommendation - check out pm 16 for greg's article.