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?
(gem) Hvis du venter på din tur kan du klikke på "ændre" ved siden af "Opdater" på hovedsiden, sætte "Automatisk opdaterings-interval på hovedsiden og spilsiderne" til 30 sekunder. Så får du vist din tur hurtigere. (Servant) (vis alle tips)