The abdominals are a very important topic when it comes to working out and a weak stomach is often overlooked. I strongly feel that the abdominals should be worked almost every day and can handle the repetitive work-load.
The lower abs, however, seem to often get the shaft when working the abs. Although the rectus abdominous is one muscle, it is often referred to as having two parts: upper and lower abs. So, most exercises mainly focus on the upper abdominus and the lower part gets overlooked. This could be due to a few reasons, such as lower back pain or just not enough core strength to incorporate/engage the lower abs.
Low back pain is often due to a weak core and tight hamstrings. I have put together a few exercises that target the lower abs. I recommend doing two sets of 15-20 repetitions.