Recommendations are earned by giving your customers a Rose from the Rose Stand when the icon appears over their heads. Recommendations are used to cook 4 Star Dishes or to unlock new areas on the Game Board. There is a 80 Recommendation limit - or so the game itself claims when hovering over the thumb icon.
There is a Lucky Rainbow that also gives out Recommendations occasionally, but it was released during the 2013 St. Patrick's Day event, and therefore most likely no longer available. Harvest it if your neighbours have one and you might get an extra recommendation.
There is also a series of Reindeer goals, where you "build" the 8 reindeer Dasher, Vixen, Cupid, Prancer, Donner, Comet, Dancer, and Blitzen. Once all eight are completed, they unlock Rudolph, who gives one recommendation and 5 Coins every 24 hours. In Jan 2015, it is currently not clear if these goals will be retired at some point.