I can't really say a good stretch for the lower back. I have some back and neck issues every now and then and the best thing I do for that is massage. You might consider getting massages from time to time. I go to Massage Envy. I don't know if you have one in your area or a place like that ... and this is not a spam for them. But you can pay a low monthly fee, which comes with one massage a month and if you do more, or want a longer one, you pay an extra fairly small fee (compared to most massages). I don't always use a massage a month, so they build up. When I'm heavier on my training, doing longer or more intense rides and/or events and need more massages, I have some built up and can go every week or every couple of weeks for a while. I have like 5 or 6 right now I think and plan to start using them more ... starting Thursday!
Secondly, one of the ways I get some stretching in is at the end of a ride, when I'm spinning to recover, which you really should do, particularly after a hard ride. I'll spend like 10 minutes spinning my legs and on a flat road, I'll stop pedaling every now and then and stretch my legs. I'll start with my calves. I'll stand and put the pedals at about a 3:00 and 9:00 o'clock position and stretch my calves, then I'll switch which foot is in front and do it again. I then pedal a little bit, and stand and stretch my hamstring. Its hard to explain how to do this, but the front leg is higher than the back by a little. I lean forward and can feel a stretch in the front leg. Then switch. I'll usually go back to stretching just the calves again and then the hamstrings again. I didn't learn this from anyone. It's just something I started doing somehow on my own.
When I come back from my ride, I usually enter my stats in bikejournal.com and, while I do that (sitting on the floor) I'll stretch some more. I'm actually doing that right now! Just got back from a ride.
Lower back is kind of hard to stretch. Depending on the sore area I might be feeling, I do try to twist and contort my body until I feel a stretch in a particular area, but I'm thinking you might find something if you did a google search. Also, if you get a massage, ask them if they can tell you a good stretch for the spot you want to stretch. I happen to go to a gal who kind of specializes in sports massage, so she is aware of areas need extra attention on a cyclist and has told me specific ways to stretch areas that tend to be tight on me. If you decide to try massages, see if you can find one that specializes in sports massage. Mine was referred to my husband from a runner friend and we have recommended her to other cyclists and runners. Ask around from other cyclists or athletes you know and see if they can recommend anyone.
Lastly, but possibly most importantly, if you haven't gotten a professional bike fit, this might help a lot with your lower back pain.