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?
(do skréše) Jak chceš vlitnót do špilo se špilošem, keré bode asi tak dobré jak te, možeš zvolet vežadované rozsah BKR v novy nabidce ke špilo. Potem nikdo, kdo do rozsaho nepasoje, toďto nabidko ani neovidi. (Katechka) (okázat šecke vechetávke)