This article is a basic checklist of what you're looking for in a streaming layout. Every stream is different. This is not a be-all end-all list of what to do, but this guide is a good way to get you started in the right direction.
Simply put, keep games, webcams, images, fonts, etc. the proper ratio of height x width. Stretching these things never pays out, and you end up sacrificing quality for filling up space.