I prefer to keep instant coffee always in the kitchen. This is generally a jar of instant coffee powder readily available. One thing that this instant coffee does not like while being stored in is - moisture, as what we get is a clumped up coffee powder which sets hard as a stone. This is almost impossible to use especially if one needs it right away.

There were multiple instances when I had to throw away or spend time scrapping or purchase in smaller quantities or just be extremely careful while handling & storing instant coffee. 

Eventually I started doing this simple trick. As soon as I opened up a fresh batch of instant coffee, I made a concentrate out of it by mixing equal parts of water to it. I stored this concentrate in a glass jar in my refrigerator. Whenever we needed instant coffee we take about 1-2 tbsp of the concentrate depending on what we are preparing say with water, milk, etc. Usually for a cup of black coffee we prefer around 1 tbsp and for a cup of cold coffee we take about 2 tbsp. It depends on how strong is the coffee and/or how strong or light do you prefer.

Please note that make sure you store it in a glass bottle or container. This concentrate should last for about 3-6 months in fridge depending on the storage conditions. Beyond that time frame take a personal call by tasting it with some water and if you get a strong acidic/sour taste, discard it. Also, I would suggest make a batch that would last for about 3 months at one go.

And that's it! My life is much simpler now.