I use a shaker for my shakes and a balloon whisk for my soup
For the shakes I add all the water I'm going to have, then add the sachet and shake really hard. It works well. I don't have lumps very often.
For the soup, I add a fair amount of water into a really deep bowl and add the soup sachet. Then I use a balloon whisk and whisk it really really well and vigorously. And keep going until they've all gone.
Alex