As Joby mentioned, use the 455 kHz beam for side imaging.
I tend to mot fiddlle with the advanced SI settings (sensitivity, contrast). I feel that the most important thing you can do to achieve the best side images is keep your SI range limited (100-125 feet max) and closely correlate boat speed with chart speed.
First, about the range. While SI's range is quite wide, in my experience it is better to tighten the range to 100-125 feet for most work, or even tighter if trying to position the boat very precisely. When the range is set too wide, objects on the sides of the boat get too compressed to be able to interpret them readily. Moreover, a very wide setting absolutely requires that you drive in as straight a line as possible, or objects on the edges will get blurred quite easily. So, tighten up your range.
Second, concerning the boat and chart speed. At my normal scouting speed (4-5 mph), I’ll run a chart speed of 3 on my 1197, and 4-5 on my 997. If my GPS speed is slower, I dial down the chart speed. Often, if I’m trolling in the 1 mph (GPS) range, I’ll crank my chart speed way down…to 1 on an 1197, 2 on my 997. These chart speeds produce the clearest side images for me. If your chart speed is too fast, your objects will become blurry and elongated; too slow, and they get too compressed. A little bit of trial and error will tell you the right chart speed for whatever boat speed you happen to be running.
I've attached a couple of screen captures to show you the detail that is possible with your Humminbird fishing system. The first shows a mixed school of crappies and white bass in a slack water area of the Mississippi River. I collected this image a couple of weeks ago. In this area, my chart speed was 1 and my boat speed was 1.2 mph. Note my SI range: 125 feet to either side. The second shows a weedbed on Lake Mille Lacs in central MN. Here, I am running from the weeds out over the deep edge at a little under 2 mph. My chart speed is 3, and my Side Imaging range is 100 ft.
If you're interested in learning more about your Humminbird Fishing System, be sure to check out all of the applications and interpreted Side Images at my website:
Instructional articles for Humminbird Side Imaging sonar units