The Job Itself: Every customer counts and is important. Never be tempted to write-off a customer as a nuisance, or "not worth bothering with." Always do a good, professional job and take pride in it, even if you lose money sometimes; believe me it will repay itself with huge dividends.
Once you start to slip into the 'cowboy' attitude then you are on a steeply sloping downward path to hack amateurism. Keep ALL of your customers satisfied, even if it means going to great lengths to ensure this.
Telephone: This is a major source of un-professionalism in even quite large companies who should know better.
Always answer the telephone in a positive cheerful manner; don't leave people hanging on and ALWAYS call people back when you say you will.
If you use a telephone answering machine, then get the recorded message done by someone with a good telephone manner. It is totally unforgivable to have an answering machine with a boring, amateur-sounding message, in the background of which can be heard a screaming baby!
Answer all messages promptly otherwise people will stop leaving them.
Stationary: This lets most small companies down. I have already pointed out the areas to watch, but do be careful of the small details - they matter.
Also watch your literacy. Very few people are as literate as they would like to think, most people are very poor at spelling and grammar. There is nothing guaranteed to put a prospective customer off buying from your company than a letter or a piece of advertising which has a spelling or punctuation error. By the way, this is a classic I-Can't! ("I can't spell, I never have been able to, I'm useless at it!") Anyone can learn to spell to a high standard; and for words which still give trouble, there is always a dictionary!
Personal Visits: Look smart and professional at all times when visiting clients or VIPs. Even if you have chosen a job where you have to get your hands dirty then invest in several pairs of overalls with your company name monogrammed onto them; don't just turn up in your old gardening clothes.
Car/van: If you need to visit clients or customers regularly then buy the very best and most appropriate vehicle which you can afford. If this is a van, then get your company name painted on the side.
If you really cannot afford a decent company vehicle then park your own vehicle 'round the corner' when visiting clients. I had to do this for a few months before I could afford a vehicle worth parking in view of a client! Clients and customers judge the success of you and your business by the type of vehicle in which you arrive - rightly so. Don't expect to pick up a substantial order if you turn up in a ten year old Ford Cortina with a coat-hanger for an aerial and two large, orange furry-dice hanging from the mirror. Even if you are a jobbing plumber this would look very, very suspect. The customer would (rightly) wonder at the quality of your workmanship if your car was in such a poor state of repair.
Book-keeping: You must ensure that you keep accurate, neat records of your transactions. Believe me, you will experience severe problems if you don't. This applies whatever the size of the business. The smaller the business, the greater the temptation to muddle through somehow.
Your business life is going to be difficult enough without having the problems of messy financial records to contend with - get it right!