The activity, through Chemistry, will include DM, in-store and sampling.
Unilever has blamed faltering company profits on lacklustre sales of the slimming plan product in recent years.
The January push focuses on its new chunky vegetable soup and Slim.Fast plan.
A "Happy 365" diary is being launched and distributed to the brand's 100,000-strong database. It contains slimming tips and money-off coupons for the products to keep the brand front of mind.
Soup samples and a leaflet highlighting the benefits of Slim.Fast will be sent out to 212,000 mums in conjunction with the National Day Nurseries Association.
To drive purchase in-store, a bespoke starter pack has been designed for Tesco stores, and 72,000 packs will be available for a reduced price.
More than half a million samples and coupon mailings are being sent out to encourage previous responders to reappraise the recently relaunched brand and prompt purchase.
The campaign is supported by online activity including editorial content on portals such as handbag.com and yahoo, and interactive ads on instant messaging sites to drive awareness and traffic to the Slim.Happy site.
"This time of year is critical for us so it's essential we have a campaign that communicates this proposition whilst driving trial and response," said Erin Smith, Unilever senior brand manager.
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