If you want to keep the sauce:
Mix the sake, mirin and sugar together and microwave for 30 seconds or bring to a boil in a small pan to evaporate most of the alcohol. Mix well to dissolve the sugar.
Add the soy sauce and garlic powder.
If you use a microwave-safe container you can just keep it as-is.
If you're going to use the sauce right away:
Mix all the ingredients together and add at the appropriate time depending on your recipe.
Recipes I've used this sauce in:
Nagoya Style Chicken Wing Karaage (Fried Chicken) (Recipe ID: 513037)
Inexpensive and Healthy Teriyaki Dumplings Using Freeze-dried Tofu (Recipe ID: 753103)
Pork Wrapped Tofu With Melty Cheese (Recipe ID: 539286)
Homemade Chicken Soboro (Recipe ID: 612108)
Grilled Sanma (Pacific Saury) With Sauce (Recipe ID: 517813)
Pork Stir Fry with Oyster Sauce and Lots of Sesame Seeds (Recipe ID: 519568)
Chicken Teriyaki (Recipe ID: 523178)
Buri (Amberjack) Teriyaki (Recipe ID: 510600)
Story Behind this Recipe
I just combined 5 ingredients I use all the time. You can add other ingredients too if you like.
By using garlic powder, the garlic flavor is distributed evenly in the sauce, which remains smooth. I also think it's a bit less stinky than using chopped or grated garlic. If you love garlic, add more garlic powder, but you may be a bit smelly the next day.