Nothing fancy. Colored maskingtape. Write with a sharpie marker. Then wrap in scotch tape. Scotch tape keeps the label from coming off and protects the ink. Have some bottles that still have this label on them after 6 months.
I bought a laminator off ebay cheap and just do the labels as i need them lot less hassle then stickers just secure them with sellotape and the labels are completely reuseable.