OAT (Overseas Adventure Travel) gives a "frequent traveler credit," stating on their website: "Every time you travel with O.A.T. and Grand Circle, you'll receive a Frequent Traveler Credit worth 5% of the advertised cost of your trip (less any discounts), applicable toward your next O.A.T. or Grand Circle trip you take within one year. Once you've completed your 5th trip with us, your the maximum value of your Frequent Traveler Credit will increase to 6% (beginning with 2016 departures)."
You can also earn credits by referring other (new) travelers, and get a discount if you pay in cash in advance (the further in advance, the more discount you get). The details are all on their website. OAT goes all over the world and I've heard good things from my friends who have gone with them. I will try them for the first time next December (Burma and Laos).
Global Volunteers, an organization that arranges volunteer trips in about 16 countries, also offers a $200 discount for "alumni" as well as credits when an alum refers a new participant. This is a completely different way of traveling that I've found quite satisfying, traveling with them a total of nine times in China, Poland, Cuba and Greece.
(Regarding the questions about the word "alumni": This word is really Latin but is commonly used in English meaning that a person has attended a particular school. Most colleges/universities have "alumni associations" that support the school. Sometimes other organizations (Rick Steves) use the term to mean someone who has participated in the past. Technically, alumni is the male plural, alumnus is the male singular, alumnae is the female plural, alumna is the female singular.)