Add an additional step into your word-lookup routine: practice it a little bit. Repeat it several times, preferably out loud and with proper pronunciation, consider the definitions a little bit, and compose an original sentence or two that incorporates the word. Personally I sometimes look up the etymology as well, that stuff just interests me in general though.
This will take a few extra minutes, but will seat the new vocab word in your memory better. It can still sometimes take two or three individual encounters to finally have the word fully remembered, but eventually it does just permanently enter your vocabulary.
You’re basically trying to force it from your short-term memory into your long-term memory just like you would with basic schoolwork, using similar techniques.