When Stella, who is 42, was diagnosed with breast cancer last year, she dreaded other people's reactions. "The thing I couldn't stand was when they cried," she says. "And another awful thing was when they trotted out anecdotes about their Auntie Gladys, or whoever, who did or didn't lose their hair.

"Usually they'd be telling you about someone of a completely different age who'd had cancer in a whole different set of circumstances. Even if the cancer was similar it was very unlikely to be the same, and it couldn't be identical because no two cancers are. In fact, it was a bit like saying, 'I had an auntie who had a disease once' - sad, but irrelevant."

Most insensitive of all, though, was the friend who told Stella that she reminded her of her cousin Jenny, adding as an afterthought, "God rest her soul." "No, no, no," says Stella. "It's not just unhelpful, it's really upsetting."

Occasionally a comment is not so much upsetting as irksome. "People are always telling me how healthy I look," says Paul d'Auria, who has had cancer for 12 years. "I tell them I'll put it on my tombstone: 'He always looked well.'" Others are annoyed by people who seek - albeit in a well-intentioned way - to downgrade their friend's illness. "You get people who say it's not so bad, at least it's treatable," says Shelley Dolan, nurse consultant for cancer and critical care at the Royal Marsden hospital in London. "But any cancer is likely to be the worst thing that's ever happened to someone, so it's not very sensitive to say it's not so bad.

"Another thing that grates with a lot of patients is friends who say, 'I know how you're feeling.' No one can ever really know how another person is feeling anyway, and there's certainly no way someone who doesn't have cancer can know how someone who does have it is feeling."

Sometimes people don't even need to speak to upset someone who has just been diagnosed with cancer. When Simon, 52, told a colleague he had got throat cancer, the man took a step back and put his hand up to his own throat. "He didn't have to say anything," says Simon. "His body language was saying it for him - thank God it's not me, and don't get too close."

Most of us wouldn't make such a faux pas, but how many of us "get it right" when someone we know gets cancer? Is there, in fact, such a thing as getting it right? Even trained therapists admit that they sometimes say the wrong thing and upset a patient - so how much harder is it for the rest of us who are not experienced counsellors?

The first thing to realise, according to Kathryn Myhill, specialist information nurse at CancerBacup, is that there is no formula for a "correct" response. You might feel shocked, frightened and upset yourself at what you've heard, which makes it doubly difficult to know how to react. But she cautions against saying nothing for fear of saying the wrong thing. "What's usually best is to acknowledge what you're feeling. Say something like, 'I don't know what to say, I'm finding it really difficult.'"

Launching into a story about someone else you know with cancer is a common response but Myhill, like Stella, says it's rarely helpful. "What you're usually doing is hiding behind these stories rather than being supportive of the friend or colleague in front of you," she says.

People who rush to offer advice are often making the same mistake. You may have some knowledge of cancer treatment or you may have once read a newspaper article on the subject, but, says Vanda Taylor, information nurse at Cancer Research UK, it's usually best to give people contacts at support organisations rather than to be prescriptive about exactly what they need to do or to present them with reams of internet research you've undertaken on their behalf.

In fact, says Taylor, what tends to be most helpful - particularly in the early days and weeks after diagnosis - is to offer to do a friend's shopping or ironing, or to cook a meal or make school lunches for their children. "People think when someone they know gets cancer they've got to make themselves available for a big heart-to-heart," says Taylor. "But my advice is to think in terms of practicalities. Someone who's been told they've got cancer still has do the cooking, buy the shopping and walk the dog - and that's often really hard when there's so much else going on. It can be a huge relief to have a friend say, 'Give me your laundry basket' or 'Let me take the kids to school.'"

She agrees that a good friend should signal readiness to talk things through, but on the patient's terms, not their own. "I generally say to people to get it out of the way early on. Say to your friend, 'I'm here for you. When you do want to talk I'm here to listen, but it doesn't have to be today or next week, it's when you want to. And I won't ask you every day if you want to talk about it, but know that I am here for you when it is the right time.'"

And for talking, says Shelley Dolan, read listening. "That's the most important thing. In our busy world, many of us are out of the habit of listening properly to what another person is saying, but that really does matter in this situation. What the other person will be saying will be so painful, so huge, so raw ... there will be silences and there may be tears, and you shouldn't rush to fill the silences or to stop the tears. Sometimes what people are saying is just so big inside them that they have to be silent for a while during a conversation, and you have to respect that.

"Avoid platitudes, too. Again, it's not easy - we all want to be able to say, 'Things will be all right.' It's a huge burden - you're maybe shocked yourself and you're not an expert, but research we've done at the Marsden shows that 88% of people with cancer turn to their family and to friends and neighbours when dealing with how they feel about it, so a lot of people are going to find themselves called on to help in this way."

Picking up on clues that might not be immediately evident is vital too, says Dolan. "You might ask someone how they are, and they say, 'Oh, fine.' And then, as an afterthought, they say, 'Well I have got a bit of pain but it's nothing much.' What that might mean is that they'd like to talk more about the pain, but they're not sure if it's OK to burden you with those feelings. And if you're open to it, you might be able to delve a bit more deeply and ask more about the pain, while being ready to back off if that doesn't seem to be something they want to go into in more detail."

Ian Young, 45, who was diagnosed with a brain tumour more than five years ago, says what counts most is sensitivity to how he's feeling, especially considering that his mood changes a lot from day to day. "What I value from friends is acceptance of how I'm feeling at this moment," he says. "Sometimes I don't want to talk about it, and other times I couldn't possibly talk about anything else. And a good friend is the person who'll say, 'Do you want to talk about it, mate?' and accept that, if you don't want to talk about it now, that's fine, but you might another time."

One thing Young recommends is to ask a person with cancer not just how they're feeling, but how they're feeling today. "People say, 'How are you?' but they don't really mean it. When someone says, 'How are you today?' they're indicating that they're genuinely interested, and also that they recognise that you are suffering from a serious illness but that your health is still different from day to day. I appreciate that."

While most of us would want to be supportive to a friend with cancer, professional counsellors say it's important to know your constraints and limits. How much you can do depends on what else is going on in your own life - if you're dealing with other major stresses, it's better to admit that from the outset rather than promising to be the Good Samaritan and then crumbling under the strain.

Likewise, it's important to be aware of sympathy fatigue, a phenomenon much recognised by cancer sufferers whereby friends typically turn out in droves to be there for them in the first month, but dwindle to single figures 18 months later when they're still battling to get to chemotherapy appointments and may be at their lowest ebb. In other words, says Dolan, to be truly supportive may mean being in it for the long haul, and it's worth being aware of that from the start.

Facing Cancer: A Guide for Family and Friends is a recent publication from the Royal Marsden, www.royalmarsden.org Lost for Words: How to Talk to Someone with Cancer is available from CancerBacup on 0808 8001234, or visit www.cancerbacup.org.uk