I have found when my back bothers me, it is because I have not stretched enough. There are exercises you can do to stretch your legs and lower back and you should do them every day if you are working dogs. It only takes a few minutes and you can do them while you are at training. Since you are the second string as far as the club helper, I think it is probably even more important since you are more of a weekend athlete than the guy doing the majority of the training.
When I make a point to stretch daily, I feel MUCH better working dogs. I agree with the other comments about working on your abbs, that certainly does help and I agree with Lisa, you do not have to do as many as the guy at your club said. I am not really a fan of back belts, seems like a quick fix vs doing the things you should be doing to stay healthy...like stretching and working on your core. If you think about the impact your body takes when a dog hits you, I think it might make sense how important it is to keep your muscles" loose" . You are more likely to get injured when your muscles are tight and more likely to hurt the next day when you don't stretch.
Since I am no longer a spring chicken, I pay alot more attention to this stuff than I used to and I am still working dogs after 33 years, so, something is working...not to mention, I'm a girl...(although girls do handle pain better than the guys ).
Recently, I am feeling problems in my shoulders, so, now I have to be sure to stretch that part and warm up now as well. I am also working on weights to keep the muscles in shape. You can look up stretching exercises on the internet. I am willing to bet you will feel 100 % better if you make a point to stretch....I know I do.
My biggest fear is hurting myself and no longer being able to work dogs as the helper. When I started doing the two things I mentioned above, the difference was more than noticible in how I felt after training.