There are no great secrets to making a website look professional, you just need to look at the result and make sure that you keep on track. Here are my favourite secret tips to follow!
Do not make it too personal! - Unless you are selling yourself as a person, for example a children's entertainer, then do not put lots of personality into the website. Too often I hear from my sales team that a customer would like a bit of this in and a bit of that. Yes, they make the sites look very personalised, but too often these take away from the professional look.
Reduce the design team - One of the biggest causes of a web site going from a professional look to a amateur look is when too many people are involved in the design. If they all read and obeyed the rest of the tips, then this would not be a problem. But at every person's review they each want to put their mark on a site and suddenly all manner of tweaks and extras are being added. Cramming all of these banners, dancing cows, splashes of colour and more onto the page that every single one of the committee have added soon destroys the look.
Watch the font size - The tiniest reduction of font sizes is sometimes all that is needed to take a site from looking home built to a professional looking website. Use relative font sizes (measured in em) rather than actual font sizes and if the visitor needs larger text they can click a button in their browser to increase it. But large font sizes that shout out to the visitor do not look professional. Sadly, quite often that means the default font size (especially in Arial font) does not look good.
Don't get carried away with colours - It may be clever to add in lots of blocks of different colours, but it does not look great. Keep to a few simple looking colour blocks nicely positioned around the page.
Use professional images - Sorry, but snaps taken on the camera of your mobile phone might look good, but buying some images from a top quality stock photography directory (I use iStockPhoto) gives such an excellent finish to the website that it is well worth the small amount that you pay.
Don't think movement is a good thing - Lots of movement is never good, not on a website. One or two small, smooth flash animations can add to the page, for example on the header, but placing lots of clever banners, snowflakes falling down the screen, light bulbs that glow and fade and so on will almost always look tacky.
In short, to make a website look professional try to not be too clever. Just because you have learnt how to add a scrolling image does not mean you have to. Use clever tricks only when absolutely needed and do not try to show off with tricks. Your website design will do the showing off for you.