If you want to sound professional, never, ever, ever use the words "excuse the pun" or "pun not intended" or even "pun intended". If you make a pun, leave it. Or find different words. Very often these "puns" are not puns at all but too-appropriate metaphors. You can avoid these by cutting out as many metaphors as possible.
Other phrases to avoid:
the aptly named (Readers can work it out.)
the former/the latter (The final element in a long list is "the last".)
Her face was a picture!
We all fell about!
(These two were popular in the 80s.)
Step away from words like:
parlance for speech patterns
proven where proved will do
not only but also (A lot depends on where you put them. “Not only verb” needs “but also another verb”.)
I came across a letter written in the way that people sometimes use when they’re dressing up their words to be more impressive—a tuxedo of prose comprising an “indeed” here, an extra adverb there, not to mention words like "comprising". (Jeff Chu, Does Jesus Really Love Me?)
In fiction, avoid: "'The old lady was becoming a bit forgetful', she thought." You can indicate a character's thoughts in other ways. And don't make the heroine "think" your favourite quotes from great literature.
Leave one space after a full stop, not two, despite what your teacher or your mother told you, 50 years ago. If you send in a piece with extra spaces some underling has to search and destroy them all. Word now flags two spaces as an error.
Typing a capital O instead of a zero, or a lowercase L for the figure 1 brands you as a dinosaur from pre-computer days.
Don't telegraph your punches. Avoid: "Joke joke joke joke", he quipped puckishly, with an ironic lift of one eyebrow. And never, ever use the phrase "quick as a flash".
Don't be afraid of repetition, but do find synonyms instead of limp "do so", "had been" or "one". For example:
It was an interesting idea, but hardly an original one.
How about "hardly an original concept"?
Don't try to be too complicated. Do keep it simple.
I know the above writing habits are common, and many books are written in this style. That’s why I can’t read them.
And remember: don't use the word "peruse" in your covering letter.
A Short Guide to Writing Well has many more helpful hints.
This Telegraph article has a lot of tips on How Not to Write.
More puns here, and links to the rest.