If you've been working for a while, you have a number of people who could be references for you: colleagues, project managers, managers. All of these are people who know what you've done.

So, how do you choose references?

1. Make sure your reference can talk on the phone. If you choose a reference who hates talking on the phone, you are not going to get a good reference. It doesn't matter what they say, if they 'erm' and 'err' and sound slow to respond, you will not get the reference you need.
2. Choose someone who can talk about the value of your work. If you choose someone who says, "Oh, yes, Tim worked here," and stops, what good is that? Even saying, "Tim was our release engineer," does not show value. Contrast that to this statement: "Tim worked on my project for 6 months. In that time, he showed me the value of continuous integration, and helped influence all the other developers into really doing continuous integration. I don't know how we would have finished the project when we did without his nudging and cajoling us into it." 
3. Choose some colleagues, and at least one manager. If you've had multiple jobs, ask multiple colleagues and multiple managers. Two or three managers and three colleagues is a good number.

If you haven't worked in the field long enough to have that many references, ask your managers wherever you worked before you got into the field. Did you work in a supermarket, or a retail store? Those managers are good choices. Did you mow lawns or shovel sidewalks? Ask those neighbors. Ask them to emphasise your reliability and value. (Yes, use those words.)

All of these ideas require that you stay in touch with people at previous jobs. You don't have to have long conversations every week. Touch base with these folks every 3-6 months, just so they don't forget you. You can even remind them you're staying in touch because you enjoyed working with them and that they'd agreed in the past to be a reference.

Remember, these people are doing you a favour. Choose them carefully, prepare them, and don't forget to thank them.

I've been a reference for many peers, previous employees, previous managers, and even clients. It's almost always my pleasure. But let me tell you about a time it wasn't.

I'd been managing a relatively junior employee who wasn't quite clear on the concept of coming into work on time. He didn't like deadlines--even the ones he gave himself. He was a little spacey, and would go off on tangential work that wasn't part of his deliverables. When he did the work I asked him to do, he was great. It's just that he did that so infrequently, he was a problem.

I'd inherited him. (Although, that was early enough in my management career, I could have hired him. I might not have known how to differentiate his talk and his actual work.) I worked with him over the course of three months, giving him feedback, and coaching him on how to deliver work on time, or at least show me progress.

Nothing worked. I asked him to find a new job. He did, in about two weeks. Ah, big sigh of relief.

About a month later, I received a call from his current manager. Could he have a reference? I asked the manager if this guy was working there. He was. Why did he want a reference from me? "Because I want to know if his behaviour now is the same as it was when he worked for you."

Oh boy. I answered the manager's questions truthfully, and added, "he's not a bad guy, he was just too much work for me to manage. If you could pair him up with someone, maybe you can straighten him out."  The manager did pair him up with a more senior person and they worked well together for about three years.

How do I know it was three years? Because my ex-employee gave my name as a reference. A potential manager called and asked me for one. I explained that I was uncomfortable giving a reference when he hadn't worked for him for over three years. "Well, you're one of his references." I replied, "I would think people would grow and expand their skills in three years, and I can't comment on those." "It's ok. Let me ask you questions." I reluctantly agreed.

Of course, what did he ask me about? Delivering completed work on time. How he worked with other people. How quickly he learned the system. How well he stayed with the task at hand. I was uncomfortable, to say the least.

If you don't reconnect with your references, you can't know what they're going to say. If you don't ask them what they're going to say, you might well be surprised. You don't have to be fired to worry about a reference from a previous manager.

Never give a reference without checking with the person first. If you're looking for a new job, talk to your references. Call them, email them if you must, but contact them and ask them what they would say about you. Make sure that the answer to "Would you hire this person again?" is an unqualified yes. If your reference has to qualify that answer in any way, discover why.

If you don't know what your references are saying about you, you are sabotaging your job search. Don't do that! Instead, reach out and connect with your references. They will be impressed. They will think more fondly and respectfully of you. And, you'll know what they are going to say. Everyone wins.