There are several sources:
- Mesillas Yesharim - You do this practically when you daven and acknowledge that you are speaking to Hashem:
הב' הוא השמחה, והוא עיקר גדול בעבודה, והוא מה שדוד מזהיר, ואומר (תהלים ק'): עבדו את ה' בשמחה בואו לפניו ברננה. ואומר (שם ס"ח): וצדיקים ישמחו יעלצו לפני אלהים וישישו בשמחה, וארז"ל (שבת ל'): אין השכינה שורה אלא מתוך שמחה של מצוה, ועל הפסוק שזכרנו למעלה עבדו את ה' בשמחה, אמרו במדרש (מדרש ש"ט): א"ר כשתהיה עומד לפני להתפלל יהא לבך שמח עליך שאתה מתפלל לאלהים שאין כיוצא בו.
JOY: The second [branch of love of G-d] is joy, it is a great, essential principal in serving G-d. This is what David exhorted us saying: "Serve G-d with joy, come before Him with song" (Tehilim 100:2), and "the righteous will rejoice, they will exult before God and delight with joy" (Tehilim 68:4). And our sages said: "the Divine presence rests on a person only through his rejoicing in a mitzva" (Shabbat 30b). On the aforementioned verse: "Serve G-d with joy", our sages said in a Midrash (Shocher Tov, Tehilim 100): "Rabbi Abahu says: 'when you stand to pray, your heart should rejoice, for you are praying to the Almighty of whom there is none like Him'".
Also refer to the Shulchan Aruch HaRav 1:1 who adds that not only davening but also learning Torah practically is a way to embody this command:
"ומכל מקום התורה והתפלה צריך להיות בשמחה, כמו שכתוב: "עבדו את ה' בשמחה
- Rabbeinu Bachya (Devarim 28:47) - You accomplish it when performing the mitzvos of Hashem in a happy manner.
תחת אשר לא עבדת את ה' אלהיך בשמחה. יאשימנו הכתוב בעבדו השי"ת ולא היתה העבודה בשמחה, לפי שחייב האדם על השמחה בהתעסקו במצות, והשמחה במעשה המצוה מצוה בפני עצמו, מלבד השכר שיש לו על המצוה יש לו שכר על השמחה, ועל כן יעניש בכאן למי שעובד עבודת המצוה כשלא עשאה בשמחה, ולכך צריך שיעשה אדם המצות בשמחה
תחת אשר לא עבדת את ה' אלוקיך בשמחה, “because you did not serve the Lord your G-d joyfully.” The Torah accuses people who do serve G-d not to have done so joyfully. A person is obligated not merely to carry out G-d’s instructions but to do so gladly, in a happy frame of mind. Joy when performing any of G-d’s commandments is considered as fulfillment of a commandment by itself, meriting additional reward. This is why one may be punished for failing to perform the commandments with a joyful heart. This is why the Torah requires that its commandments be performed with full intent and joyfully.
Also refer to the Pele Yoetz 198:28 for a similar approach.
So we see that these two first methods essentially are spiritually based. If we make efforts to work on our tefillah, talmud torah and kiyum mitzvos (prayer, torah study and fulfilment of the commandments) this is a sure way to embody the concepts of עבדו את ה' בשמחה. I saw on p.104 of Rabbi Twersky's The Enemy Within (Shaar Press: 2002) in his chapter entitled "Finding Joy in Life" that the two are inherently linked:
Rabbi Samson Raphael Hirsch points out that the Hebrew word same'ach (= happy), is closely related to the word tzome'ach (= growing). This is because human happiness is contingent on spiritual growth. If we fail to feel the joy of mitzvos, it is because we are unaware that mitzvos make us grow.
- Sefer Chasidim - One final way to practically achieve this tenet is to use it to frame your life view. The Sefer Chasidim sees it is a balancer, keeping us in check. In other words, you accomplish it by ensuring we have the right outlook on life, synthesising our view on the present together with our view of mortality.
עבדו את ה' בשמחה (תהלים ק ב) וכתיב (תהלים ב יא) עבדו את ה' ביראה הא כיצד אם אדם שמח מאד יזכור לו יום המיתה זהו ביראה ואם הוא עצב ישמח לבו בדברי תורה דכתיב (תהלים יט ט) פקודי ה' ישרים משמחי לב:
It is written, “Serve the Lord with gladness” (Ps. 100:2)1 and “Serve the Lord with fear” (Ps. 2:11), but how so? 2(both fear and gladness?) If a man is over-joyful, let him recall the day of death,3 this is fear. If he is sad, let him rejoice his heart with matters of the Law,4 as it is written, “The precepts of the Lord are right rejoicing the heart” (Ps. 19:9).