Wishing someone good luck on their future endeavors is a friendly and polite gesture that can be done in a variety of ways, preferably by sending a handwritten note. The note may be accompanied by wishing the recipient well either in person or on the phone, depending on the relationship to him or her.
According to Emily Post's website, the best way to send a message is through a handwritten note. Handwritten notes, whether written for the purpose of personal correspondence, gratitude or wishing a person well, are exciting to receive and show that the sender has gone above and beyond their social obligations by taking extra time to honor another person. Depending on the sender's relationship with the recipient of the well wishes, he or she may want to take the time to call the person or give well wishes in person too.
As is shown in the good luck letter formats on Letters.org, the letter should be kept brief, should focus on the recipient's hard work and preparation and should keep the tone positive. By keeping the focus on the person's work in preparing for the challenges ahead and congratulating him or her on what they have already achieved, the sender is more likely to avoid sounding false or trite. Well wishers should keep in mind that simply saying "good luck" may seem to suggest that the person's success depends solely on luck. As noted in the Huffington Post, this can be more of a turnoff for some people.