Nick,
A good idea is to do some progressive stretches immmediately AFTER you work out(this is very easy to overlook because most folks figure when they are done working out, they are done for the day). You can find a book of the stretching routines at just about any library; I bet that you will find that your flexibility improves quite quickly if you are otherwise fully warmed up.
From what I have observed, the thing about improved flexibility is that it requires a continued commitment and gradual is almost always better. In other words, better to do some every day, than try to do a lot once or twice a week.
OBC