Labour would refuse to accept "no-go areas" around the country in the run-up to the general election campaign, Harman insisted.
The deputy leader wrapped up the party gathering in Manchester with a speech suggesting the opposition could devote resources to unwinnable seats to pursue Ed Miliband's 'one nation' vision.
"We've got to have no no-go areas for Labour. Because people all over this country are suffering with this government," Harman said.
"To win the next general election we must – all of us – adopt a marginal seat mindset and listen to the people where we don't have Labour MPs as well as where we do.
"That's why every one of our shadow ministers will adopt a marginal seat – working alongside our Labour candidate, to listen to and understand the concerns of people there."
Shadow chancellor Ed Balls will work with Clair Hawkins in Dover and Deal, Harman revealed. Shadow business secretary Chuka Umunna will assist Norwich's Labour candidates Clive Lewis and Jessica Assato.
"We've got to reach out beyond our party faithful into communities, connecting with people who otherwise feel that politics has nothing to offer them," Harman added.
The Labour veteran joked that she was frequently confused by members of the public with Tessa Jowell and had read erotica classic 50 Shades of Grey "for research purposes".
"I have to say I don't think it's very realistic," she said.
"Because, let's be honest, what most women want is not a man who ties you to the bed, but one who unstacks the dishwasher while you watch the Great British Bake-Off."
Ed Miliband has changed his mobile number to avoid receiving further texts from the Liberal Democrats, Harriet Harman has confirmed.
Labour's deputy leader sought to present the move as an anti-Lib Dem joke in her speech concluding the opposition party's conference in Manchester.
It follows a claim last week by a member of Labour's national executive committee that party staff had given their leader a new mobile with a new number.
"It's no wonder Vince Cable is on manoeuvres. But let's not forget Saint Vince is in it up to his neck too," Harman told conference delegates.
"After all, it was his policy to treble tuition fees.
"So I have a message for Vince. Don't bother texting Ed - he's changed his number."
The Conservatives will be Labour's main enemies in 2015 as both parties struggle to achieve an outright majority. But Nick Clegg's Liberal Democrats will also be targeted as Miliband seeks to enter No 10 in his own right.
"No one deserves Nick Clegg - Calamity Clegg who has propped up this miserable Tory government every step of the way," Harman added.
She said this year's conference had fired the "starting gun" on the 2015 general election and that Labour now had a "fighting chance" of forming the next government.