Lately, I’ve found myself giving the same design feedback over and over again to my friends and colleagues. So, I decided to write down the basics, which should help anyone tackle the most common pitfalls.
1. Leave space

One of the most common errors I’ve come across is elements placed too close to each other or the edges. Whether designing headers, images or body text, be sure to have enough space around the objects. How do you know how much is enough? Unfortunately, no fundamental answer exists, but for texts, for example, leave at least one cap height of empty space around the text. For images, defining the right distances is a bit trickier, but aim to have “balance” and “sense of calmness” around the image — so that it is easy to look at.
2. Be consistent

The point of design is to ease the job of the reader and help them to understand the information shown to them. By being consistent, the reader doesn’t have to figure out the quirks of your design again and again. The reader knows what to look at immediately when (s)he sees your design. This means, once you decide to have your headers with some font, use that same font for all headers. If your logo is in the lower right corner in your slides, keep it in the same exact spot throughout the slides (DO NOT let it dance through the slides).
3. Align items

This is by far the most important one among all of these. If the elements are not aligned, the design feels cluttered and is difficult for the reader to follow. Just like anything that is unorganised, unorganised design is hard to comprehend. In addition, unaligned objects easily give an amateurish feeling to your design. So keep your ducks in a row and Bob’s your uncle.
4. Give guidance

When designing, always think of what is the most important piece of information in your design and give the most importance to it. You can assign importance e.g. by colour, size or position. This works the other way also; for elements that are less crucial, lower their importance by reverse means. By bringing contrast into your design, you avoid potential dullness and your design is enjoyable to look at.
5. Group

Elements that are related to each other should be grouped together. Grouping creates a rhythm to your design and allows the reader to scan and skip information faster. The enhanced structure helps the reader to find the information (s)he is looking for faster and more accurately.
BONUS
One might argue that by following these simple rules, designs still lack the professional feeling that you get from carefully crafted designs. While the basis of professional feeling comes from the rules above, adding extra graphical details are required to make the design come alive. This is definitely something only learned through practise, but a good example would be adding a simple line as in the image below.

To come up with these small lively details, one should copy with pride or at least search for inspiration. Fortunately, the web is full of great sources of inspiration; Dribbble, Behance, Smashingmagazine and Designspiration are good places to start your journey.