Soldato
I use garlic and onion powder in my cooking and I've always found that they both rapidly turn into rock hard lumps, meaning either I have to slice bits off to use, or bin and open new ones! I've seen a load of things to try, rice, cornflour, air-tight tubs, but none of them seemed to work. I now have the answer. Store them in the freezer! I've had a jar of each in the freezer, opened, for well over two weeks now and they both pour as easily and freely as when I opened them! I'm a convert! 
Anyway - the tip works!