For every business there is one other company you are compatible with. If you have the farsightedness to connect with at least one of them you can split advertising costs, cross promote and freshen the way current customers perceive you. One of the biggest benefits is of course the opportunity to tap into each others customer base and reach a broader market. For the best results this partnership should be done for a specific period of time. 

So stretch your imagination, think about the lifestyle of your customers and the time of year  

Holiday Season: People are looking for gift and entertaining ideas 

Coffeehouse + Boutique liquor store that sells liqueur & brandy  

Bakery + Specialty Tea Shop

Spa + Bookstore

Craft Store + Home Decor Shop

New Year: Clients want to set new goals in motion 

Life Coach + Professional Organizer

Gym + Dietitian

Health Food Store + Alternative Health Practitioner

The equations are limitless; providing an opportunity to test the waters for different demographics and ad mediums. Do your research though and make sure the company has a compatible level of customer satisfaction and their business ethics match your own.