I think one of the biggest mistakes made when playing black is to take your eyes off the king. Its easy to be distracted by other things happening, so the king sneaks away!
As white, probably one of the biggest mistakes is to lose too many pieces early on. You have less pieces than black so have to use them strategically.
Maybe one of the better players can go into this in more depth?
(kaŝi) Se vi volas ludi kontraŭ ludanto de simila nivelo, vi povas difini intervalon de BKR ĉe invito al nova ludo. Tiukaze neniu kun BKR ekster tiu intervalo povos vidi/akcepti ĝin, (Katechka) (Montri ĉiujn konsilojn)