Several of the cases have been looked into by police, but none has led to a prosecution because the authorities have the power to use their discretion.
But earlier this year, Law Lords said Debbie Purdy, a multiple sclerosis sufferer from Bradford, had the right to know whether her husband Omar Puente would be prosecuted if he helped her travel abroad to die.
Ahead of the publication, Mr Starmer said: "It is not easy and there are lots of factors that have to be taken into account.
"The basic approach we have taken is to try to bring some clarity, but at the same time to protect the vulnerable."
Mr Starmer said there were a number of key factors that would be considered in a prosecution.
These included whether the person stood to benefit financially from the death, whether they had encouraged the suicide for reasons other than compassion and whether the person wanting die was considered competent enough to make that decision.
But Mr Starmer said there would still be a degree of flexibility as every case he had reviewed in recent years had been different.
"If we put it in a straight-jacket it will cause more problems than it resolves."
The guidelines will come into effect immediately, although this will only be on an interim basis while they are consulted on.
Interview with Debbie Purdy
Sarah Wootton, chief executive of the Dignity in Dying campaign group, agreed more clarity was needed.
"This will represent a significant breakthrough in our campaign for greater choice and control at the end of life."
But Dr Peter Saunders, of Care Not Killing, an umbrella group of doctors, religious organisations and charities, said it was a shame the DPP had been forced into this as the current situation was "working well".
"We hope no-one will be given immunity. It is only right that each case is looked into as this acts as an effective deterrent and protects the vulnerable.
"There is also a danger that the DPP will become a consultancy service for law breakers."
David Witt, from Tonbridge in Kent, who took his wife Elaine to Switzerland to die two years ago when she was suffering with lung cancer, said the lack of clarity had made it difficult.
The 64-year-old said: "[The guidelines] would have helped us a lot. On top of all the physical and emotional problems, there's this uncertainty. You can't tell people what you're thinking."
Anyone assisting someone to commit suicide can face up to 14 years in prison under the 1961 Suicide Act, which covers England and Wales.
A similar law applies in Northern Ireland and fresh guidance is also expected.
In Scotland there is some uncertainty as there is no specific law on assisted suicide, although in theory someone could be prosecuted under homicide law.
A bill is expected to come before the Scottish parliament soon in a bid to legalise assisted suicide.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.