You beat me to it Kai!
I use a refractometer regularly when mashing and boiling. Evaporation is a key concern when sampling. I use a dropper and sample quickly. Clean your dropper with distilled water and ensure it is dry upon each use. Apply several drops to the plate and cover. Allow 15 seconds for the tempeature to equilibrate.
Make sure you calibrate your refractometer with distilled water prior to use to ensure proper calibration. I usually take multiple readings to ensure repeatability. Assure that there are no visible solids or bubbles on the daylight plate.
Always clean the plate with warm water and dry with a clean soft cloth after each use.
A refractometer is a great way to qualify and confirm your mash is converting as well as determining the amount of sugar that is in your wort. I won't brew withoout it.