It's the most basic design principle. There's no excuse for not remembering it. Yet I see it being violated over and over again. So, as a reminder, here we go again:
If you want anyone to be able to read your advert, website, presentations etc., it's important to ensure there's good contrast between the message that you want people to pay attention to and other noise around it so that your message stands out. This can usually be achieved by using appropriate colour contrast, image contrast, text size, image size, plus taking into account the medium where the message will be presented: banner advert, website, mobile phone, magazine etc.
If you are using a certain style in your design, try to keep the consistency by repeating the same style across the design. Especially on websites, repetition aids scanning (which is how most people browse websites by the way), so don't present your text in various styles all around your website. For example if you use a dark blue font size 48 for all level one headings, repeat that across the site. It also makes a website look less cluttered and clean
Make sure everything is aligned and organised. Text, images, form fields, bullet points ... Nothing is more annoying than having to dart your eyes around the page while reading something. It also reduces the effectiveness of the message if information is not aligned properly. So what's good alignment ? It depends really. For a long passage of text, left alignment makes it easier for reading from left to right. Central alignment works well for title headings when used properly, but centrally aligned text is just too hard to read.
What belongs together stays together and vice versa. Elements on a page which are related to each other should be grouped together and likewise, elements that don't relate to each other shouldn't be placed in close proximity as this might confuse its readers. That sounds straightforward but is not always easy to apply to website designs. Hence taking user tasks into consideration is also important in making decisions on organising page elements according to relevant proximity.
If anything, the above principles should be used as a magic mantra the next time you hesitate where to put that little box (or anything else) on a web page. Just chant C.R.A.P. and make sure that all 4 principles are fulfilled before the little box finds its home.