Well I will quickly run through a list of common mistakes which people new to designing make. They can easily be fixed and will make your designs look so much better. I have also included some examples which I have made many years ago 
Bad choice of colour. The most common mistake which makes designs look horrible is using bad colours. The most common mistake is to use extremely bright colours. Other bad mistakes is using black for background all the time. The use of #000 makes the design look extremely dark and really highlights the brights colours causing the design to hurt your eyes.
Terrible choice and use of font. This one really bigs me when I look at new designers work. When designing stick to the basic fonts: Arial, Trebuchet MS, Verdana etc. Also when designing in photoshop do not put anti-aliasing on content text. This makes it look horrible and is not available if you coded the design. (Anti aliasing is making font strong, crisp etc, set to none).
Making everything huge!. This is a common mistake and can also be made by making everything too small. A quick guideline I would give is to make you content text 11-12pt in size. Often it just takes a while to get the sense of scale when designing. It should hopefully begin to come to you if you keep designing. This mistake can also be made if you change screen resolution.
Using outer glows and shadows all the time. A lot of people seem to do this at first. They make text or objects have severe and very noticeable shadows or outer glows. If you are going to use them please keep them as subtle as possible to avoid them ruining your designs. When I look at the banner - which I have a preview of - I would actually think it would be good if it wasn¡¯t for the outer shadow.
Over use and too severe gradients. Again this is a case of being subtle. When you add a gradient you must keep it subtle otherwise it will just stick out and look strange. Use gradient in different directions can also make a design look pretty strange so try and be consistent. This is a mistake which appears to be getting more popular. Gradients can be great but only if they are used at the right time and are using the correct colours.
Making the designs almost impossible to code. This can be a result of using too many gradient in different directions. When designing I like to on change something in one axis - x or y. That way you can use css when you code it and use repeat-x or repeat-y.
Thinking your designs are great. Last but not least this problem is psychological and will hold you back. Listen to what people say when they give you tips on your designs and remember; not everyone has the same tastes.
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