There's this exercise I do that involves me getting on my hands and knees and lifting my legs. It has really helped my lower back, alot.
If you are looking for a more detailed help there's this wonderful site that can help you.
http://hyperstrike.com/
It offers lots of exercise and tips how to do it right and also an animation showing you how to do it. Also you can choose what you're looking to work on and it gives you lots of options for different exercises.I recomment you taking a look at it.
I really like how you can even look for certain exercises that involve equipment you already have, even a rolled up towel
