I've noticed a big difference when I started doing more lower back exercises, makes the abs work out pay off way more.
Hi, I would suggest you to do the basic exercises like squats, death lifts, pullups rows, power cleans etc. which activate the big muscle tissues together and take a lot of energy consumption. The best way to have slim body is Fullbody or total body training. See it on internet if you want to know the benefits of that training style.