Stretching and warming up is important and here's a tried and true method:
Bridging helps stretch your back and strengthen it all at the same time. What you do is lay on you back than arch yourself upard supporting yourself on your hands/head and feat. This a great effective exercise that's been in action over the years for one reason it works, and for a lot a reasons. In your case it'll take the load off your spine allowing it and the rest of your back to relax. You can do it with or without equipment.

One exercise I have found to helps work the lower back is knee/leg raises off a chin-up bar. This is one tough exercise it's one of the best exercises for the abs in conjuction with your back which acts as a stabiliser, this also takes a load off your spine letting you build much needed strength at the same time.