Here are a few general tips.
Put things in WRITING with your clients to CYA. This includes, but is not limited to estimates/quotes and warranties, as well as services rendered. Also, a record for them that you have their system, and which someone else can use (other than their word) to pick up the system, and a record for you so you know who to phone when it is ready.
Your time is worth money. Don't get into the habit of giving away free labor. You do it once, and that person will continue to expect to pay the same.
Be timely in your repairs, and keep communications open. People hate to wait weeks for a simple repair, and hate to not be told their system is ready. Give time estimates of WHEN repairs will be completed. If in doubt, give yourself more time. Think you can finish repair in 2 days but aren't positive? Tell customer 3 days. That way if something happens (power goes out, surprise visit from in-laws, whatever) you're covered.
Customer data is very important. If you ever have to do anything that will wipe data and customer says it is okay, put it in writing and get their signature. Nothing worse than them coming back to you saying "you deleted my wife's photos."
If at all possible, power on the machine in front of the customer and have them show you what is wrong. This does two things. One is that it clarifies the problem because sometimes their description does not pinpoint the issue. Two is that it verifies there isn't something else wrong with the system. You don't want to be handed a system for a simple RAM upgrade only to find out after the customer leaves that the system does not power on, and the customer swears up and down that it worked fine before you touched it.
Make it clear that payment is to be received when work is done, or else they don't get back their system. If you let a repaired system leave your door without payment, kiss those hours you put in it goodbye. Most people are honest, but not all.
Don't try to be the cheapest. There is always someone on Craigslist that will undercut your prices. Differentiate yourself by doing better work and being more professional.
Don't rip people off. We've all heard of the mom&pop computer shops with outrageously high prices. Only takes one time for a customer to find the same wireless router at Best Buy for half the price, for that customer to become a former customer. Make a reasonable but not rip-off profit on labor, and don't worry about making much money on parts.
Don't lie to customers. If you don't know something, then you don't know. A customer that catches you in a lie is an ex customer. It is better to say something like "I don't know, but I can find out."
Consider a cheap phone to give out for business calls. Last thing you want is after hour calls asking how to check their email. You can get something like a Magic Jack or other cheap VoIP service and use it exclusively for your side business.
If your volume is high enough or you want to start servicing business clients, consider becoming an "official" business. That may entail filing a fictitious name (name of your business) with the city, getting a bank account for the business, getting a tax ID for the business, as well as keeping records and paying taxes. You're supposed to do all this anyways, but it becomes more important with higher profile/volume or business clients.
Don't badmouth competitors. That just makes you look unprofessional.
Don't be the guy that says "I only use these because the other is crap." Computers are not sports teams. I know it is popular to take sides in enthusiast tech forums, but in real life if you badmouth a brand or part that a customer just spent a lot of money on, they are likely to find another tech who is willing to work with their stuff. Also, there is often no one part that is universally better or worse. For every user in these forums that say "I only use brand A because I've had nothing but problems with brand B," you can probably find another user that says "I only use brand B because I've had nothing but problems with brand A."
Facebook
Twitter