The best way to have professional looking promotional material is to employ a professional. However, if for whatever reason, you want to design your brochures in-house, then here are some tips that I hope you will find useful.
1. Control your font usage
Just because you have loads of fancy fonts installed on your computer it doesn't mean you have to use them all to make your page look interesting. Decide on 2 different font families - one for headings and one for main text. Choose contrasting fonts types. For instance you could use a serif font for text and a sans serif for headings. (Serifs are the pointy bits at the ends of the letters. A typeface that has serifs is called a serif typeface, a typeface without serifs is called sans-serif, from the French sans, meaning "without"). You can vary it a bit, however, by using different styles (i.e. bold, italic etc) within the same family.
2. Use appropriate fonts
Typefaces unconsciously convey mood so, depending on the content, use a font that is sympathetic to the subject matter. For instance Comic Sans would be ideal for a baby clothing catalogue. I'm just using Comic Sans as example here because you are probably familiar with this font. I am not necessarily recommending it, as personally I hate it!
3. A picture is worth a thousand words
Illustrate your narrative with appropriate pictures. If you can get away with a generic image, use stock photography. There are many microstock photolibrary websites on the internet where you can buy very professional photographs for just a few pounds. The one I prefer is Dreamstime.com. Pay a few more pounds and get a high resolution version - if you buy the web resolution (72 dpi) it will look fine on a monitor but terrible when it is printed. Icons, Illustrations etc are similarly available but be careful that you use the same style of graphic throughout your document. Nothing looks more amateurish than lots of different styles of of clipart.
4. Less is more
You don't have to fill up every square inch of the page - unless, of course, you are designing a newspaper! Use the white space creatively. A dense block of type will put people off. So cutting down the words and opening up the "leading" or line spacing will give your publication a more modern, cleaner and easier-to-read look.
5. Add a bit of variety
Break up the rigidity of your layout with a few quotes or testimonials in a larger type size or put a few interesting facts in a tint box or "side bar".
6. Nail your colours to the mast
Try not to use lots of different colours. Restrict your palette to a few "corporate" colours that reflect your company branding. Use black for your main text and use the other colour(s) for headings, sub headings rules etc.
7. Do it in style
Professional publishers use style guide to ensure consistency throughout their printed media. You don't have to go that far, just have a few simple rules about when and when not to use capitals for instance. The trend these days is to use less capital letters and less punctuation. So avoid excessive exclamation marks!!! (like that) and take out all the double spaces after a full stop. That may be correct for typing but not typesetting. Also while we are on the subject of consistency, decide upon a font and make it your "corporate font" and use it for all you publicity material - this will reinforce your corporate image.
8. Widows and orphans
These are short lines at the beginning or end of a paragraph, which are left dangling at the top or bottom of a column, separated from the rest of the paragraph. In other words, when a paragraph runs into another column or page make sure there is a minimum of 2 lines at either end of the paragraph. The Chicago Manual of Style defines a widow as, "a paragraph-ending line that falls at the beginning of the following page/column, thus separated from the remainder of the text" and an orphan as: "A paragraph-opening line that appears by itself at the bottom of a page/column."
9. Size matters
Unless you are designing a book for visually impaired people or a children's picture book, don't set your main text above 12pt. In fact personally I would never go above 10pt. Also, if you are using say 9 or 10 point, then set the pages into double column rather than have the text stretch right across the full width of the page otherwise you may end up with single line paragraphs which look odd.
10. The proof is in the reading
Finally use the spell check facility but don't rely on it - it's not fool-proof. Get as many other people to proof read your copy. Don't just do it yourself as you can become word blind. When we read, we scan the words in blocks, we don't look at the individual letters that make up each word and we anticipate what comes next, so try reading backwards and look at each word without trying to understand the meaning. Also it's very easy to overlook the big headlines and just concentrate on the main body text, so don't forget those headlines.
Always ask your printer for a final proof before printing. Remember once it's printed it's too late!