One protocol is to add sugar to your buffer at 1-5% level. Sucrose is commonly used. Trehalose is more expensive but may give better product stability. The are more complex formulations which you'll find by looking at any freeze drying methodology (I appreciate you probably won't do freeze drying here) but the simplest thing is to add sugar and do some stability tests with and without sugar at varying levels. To find the best buffer also requires you to do some trials, but I'd keep it simple and use a common lab buffer at around neutral pH, probably quite weak (10-20mM). You'll have to experiment with salt addition and check whether it helps or hinders. Presumably you will drive most liquid off by centrifugation, in which case 150mM salt may be fine.