Most obviously, Ullr is a god of skiing and winter sports, and as such he is most often called upon for safety while zooming down that snowy mountain. He’ll make sure you don’t break your ankle, or your head. He can also be called upon for safety (or winning) in skating or hockey. In addition, he is called upon for luck in hunting, at any time of year but especially in winter. Ice-fishing is another place where Ullr’s help may be called in.
Aside from his more athletic traits, Ullr is one of the Gods who witnesses oaths. You can find his oath-ritual on this site.
In order to repay him, you can donate time or effort to his special charity, the Ullr Foundation “Ski For Light” charity, which acquires winter sports equipment for disabled adults and children.