Worth clicking Eric Barker's source link above for details.

I summarize them here:

1. Thank someone. First thing in the morning. By email.

2. Spend money on someone else.

3. Give 5 hugs.

4. Work on something you're good at.

5. Do 5 acts of kindness for others.

6. Create some fun plans to look forward to.

7. Spend time with friends.

8. Before bed, write down three good things that happened to you.

More on thanking someone first thing: 

http://www.bakadesuyo.com/2012/04/what-are-quick-little-things-you-can-do-to-in/

Number 8 (write three good things) "has been proven again and again and again." 

Here it is, explained by its originator, UPenn professor Martin Seligman in Flourish:

Every night for the next week, set aside ten minutes before you go to sleep. Write down three things that went well today and why they went well. You may use a journal or your computer to write about the events, but it is important that you have a physical record of what you wrote. The three things need not be earthshaking in importance (“ My husband picked up my favorite ice cream for dessert on the way home from work today”), but they can be important (“ My sister just gave birth to a healthy baby boy”).

I also really like the hugging advice:

"People assigned to give or receive hugs 5 times a day ended up happier than the control group."

From Sonja Lyubomirsky’s very interesting book, The How of Happiness:

In a one-of-a-kind study, students at Pennsylvania State University were assigned to two groups. The first group was instructed to give or recevie a minimum of five hugs per day over the course of four weeks and to record the details. The hugs had to be front-to-front (nonsexual) hugs, using both arms of both participants; however, the length and strength of hug, as well as the placement of hands, were left to their discretion. Furthermore, these students couldn’t simply huge their boyfriends or girlfriends half a dozen times; they had to aim to hug as many different individuals as possible. The second, the controls, was instructed simply to record the number of hours they read each day over the same four weeks.

The hugging group (which partook in an average of forty-nine hugs over the course of the study) became much happier. Not surprisingly, the students who merely recorded their reading activity (which averaged a not-too-shabby 1.6 hours per day) showed no changes.

Michael Norton's TEDx talk, "How to Buy Happiness":

These seem at first almost impossible to do.  But I bet after you do them a couple times the gratitude will be forthcoming!