Don't work for free, and don't get a reputation for working for free.
That being said, be helpful. Especially with repeat customers or when doing a large job. Take 5 min to help them lift something, or hold something they are putting a bolt into. Don't spend hours helping for free though, or it will become expected and word will get around.
This is sort of related. there is a farmer that I help out from time to time. He scraps, but doesn't mess around with small stuff. I give him scrap advice, and share my connections. He gives me some of his "clutter": that isn't worth holding onto. If I stop by to say hi, i help with what he is doing for a few min, or if he calls and says he needs a hand for 30 min, I won't take any money. He has already done a few favors for me, and I am sure if I called him right now, he would load up some tools and meet me anywhere.
Respect other people and their time. Also respect yourself and your time.
also, if you are changing plans so you can meet a deadline, tell the person. I usually say "I'm busy that day, but I think I can move things around. Let me call you back later/tomorrow" That way they know that not only are they important, but you are consulting your other client before changing plans.
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