Here are some mistakes that make you look very bad.
1) forgetting commas around appositional phrases:
WRONG:
“The book, as we have seen is long.”
“The last time I, dressed in green in red attended a party…”
“Stanko and Janko, the well-known gangsters both forget where they had parked.”
CORRECT:
“The book, as we have seen, is long.”
“The last time I, dressed in green in red, attended a party…”
“Stanko and Janko, the well-known gangsters, both forgot where they had parked.”
2) Know the difference between “first” as a dubious synonym for the adverb “firstly” and “__ first”
“First person to set foot on the moon…” is embarrassing.
Less embarrassing but still wrong:
“There are three ways to eat a chocolate eclair. First…”
No. It has to be “THE first” because you mean “The first way…”
3) Not “a bit snotty way” but:
a) “a bit of a snotty way”
b) [more elegant-sounding and formal: “a somewhat snotty way”
ALL OF THE TIPS OF THE YEAR ARE AVAILABLE AT:
http://www2.arnes.si/~bjason/LW%20tips.pdf