hi mate a good one for this is to lie face down on the floor with your arms out as if you had a bar across your shoulders.
Then gently lift yourupper body from the floor using your arms and pushing your pelvis toward the floor. you only need to raise a few inches at most, hold and back down.
Once you get good with this then try without the help of your arms, but not too start with, you will feell it straight away but go slow.
Sit ups are also a major one for lower back
good luck
Paul