Very simply - being able to ask questions and have them answered clearly; several doses of Rescue Remedy; not being dismissed over concerns and allowing the possibility that complementary therapies can provide useful adjunctive relief (aromatherapy, nutrition, aloe vera etc.)
My mum died of Ca colon - we weren't told that she had it or was terminal until 8d before she died, although we believe she knew and didn't want us told. The doctor who told us was very good at the time but then distressed Dad later by looking straight through him as if she'd never met him when he tried to talk to her. However, the hospital where she died (she couldn't be let home, despite her desperate wish to be) did kindly move her to a single room in ICU, and bent the rules a little so my sister's children could come and say goodbye.
My Dad has had Ca prostate - Royal Marsden have been fantastic and continue to support him with check ups; they have a good support group there too, I believe. He is technically clear of it but his PSA has just started to rise again so they're keeping a watchful eye on him. They were happy for him to use aloe vera on his radiation "burns". The patient support group has been an utter godsend to Dad - when he started to get really tired after his 5th radiation treatment (having felt fine before then) he tried to fight it, but one of his support group told him off and explained that it wasn't like normal tiredness and you need to give into it and rest. He did, it helped.
My niece had a brain tumour on her cerebellum when she was 21mo. Well she probably had it before then but it took 3-4m before it was diagnosed. Her GP thought that my sister was neurotic, or that my DN had an ear infection; the hospital were clearly irritated and told her that the problem was either tonsilitis or teething 
. It took my sister taking DN up to UCH paediatric A&E to get the right response, by which time my DN had lost the ability to walk because her balance was shot, and was fitting occasionally ("febrile convulsions", said the GP, despite there being no fever). Luckily DN had the tumour removed (well, 98% of it) and is still checked every year - she's now 8 and doing well.
I (unfortunately) forget who it was that helped my sister with the DLA / carer forms, but whoever they were, they deserve a gold star for taking on the job.
I need to be checked to make sure my bowel is in good order, because of Mum. But in the meantime my GP made sure that my vit D levels were checked because there is an association with low vit D and cancer risk, especially bowel cancer (and infertility and MC is associated with low vit D too). There needs to be more awareness of vit D in the UK; its associations with health/disease and the fact that most people are low in vit D now because of the way they live, the sunblock society, the lack of it in the diet etc.
I was below normal range, so take 1000 IU vit D3 daily. And eat oily fish and get a bit of sun (but not too much, I'm now in Australia, it's dangerous).
Knowledge of possible preventative measures for family members of cancer sufferers is helpful to some (like me) - then we can take steps to help us avoid it as much as possible.