A good visual aid is something that will help people remember the 5% of the talk you really want them to recall.
Great questions to ask as you prepare:
1. Is there a story I can tell? Jesus told great stories.
2. Is there a quotation I can use? Jesus used many quotations. Sometimes positively (e.g. when tempted in the wilderness); sometimes negatively or to expand upon - 'You have heard it said... but I say...'
3. Is there a picture I can paint? Paul uses many images in his seemingly dry passages.
How would you illustrate:
God's justice (Romans 2:1-2)
Sin (Romans 3:23)
Redemption (Romans 3:24)
God's love (Romans 6:23 or 8:38-39)
If each of your main points has a story, a quotation and a picture you are doing well. You could also add a question. It also means you can omit one of them if you're running over.
4. Which sense am I appealing to? The best order of memory triggers is probably:
Taste, smell, touch, sight, hearing
The order a sermon uses is usually:
Ancient Proverb (it may or may not be Chinese)
I hear and I forget
I see and I remember
I do and I understand.