During my critiquing of works on this site and others, I've noticed a near epidemic of two particular mistakes I'd like to call attention to.

Well, 'mistake' might be a little strong for this first one, but I do feel it's something writers should consider when trying to make their prose sound more professional. When writing dialog, writers feel the urge to tag every line so it comes out something like this:

"I don't know why you had to go and do that," Sage said.
"What do you mean?" Ricky asked. "I thought the body was supposed to go in there."
"In the refrigerator?" Sage said.
"Well," Ricky said, "It kept down the smell."

Or worse:

"I don't know why you had to go and do that," Sage said angrily.
"What do you mean?" Ricky inquired. "I thought the body was supposed to go there."
"In the refrigerator?" Sage responded.

They overuse 'said' or, even more distracting, they overuse its synonyms. 'Said' is generally an invisible word, which is why it can get away with being used often—which is why this sort of thing technically isn't a mistake. The real problem starts because each line of dialogue, plus the tag, tends to have a similar sentence structure, so it all starts sounding the same. Also, some lines stand best on their own: 'In the refrigerator?' should speak for itself, and tagging it as Sage's line just stretches out a line that is better concise.

And then there's the habit of having a person get two dialog tags to one exchange.

"I think you're making a mistake," Sage said. "For one thing, if you ever wear polka dots to Prom I promise you will be laughed out of every social function you every try to attend, ever," she warned.

My guess is that the writer forgot they tagged the dialog, so they do it twice. Or maybe it's a fashion statement. I find it looks odd.

You can drop dialog tags when:
-You mention the speaker already in that paragraph with an action beat.

Sage put her hands on her hips. "I think you're making a mistake," she said.

could be

Sage put her hands on her hips. "I think you're making a mistake."

-There are only two people in the exchange. Tag dialogue about once every 3 lines, so the reader can keep track, but you don't need to tag every line. If someone responds to Ricky's comment, it's probably Sage, because she's the only other person in the room. Obviously, if a new voice interrupts their conversation, it should be remarked on.

I find the dialogue I enjoy most in published fiction either mentions characters' actions with their speech or, on occasion, tags dialogue with strong, descriptive synonyms of said, like 'muttered', 'called', 'accused', 'rasped', or 'demanded' (spicing 'said' with adverbs is weaker and usually takes more words than you need—'she said quietly' could just as easily be 'she whispered' or 'she murmured'. Spicing synonyms with adverbs—'she whispered angrily'—should be done rarely, and only when the aspect the adverb describe isn't immediately obvious).

The second mistake I see really is a mistake: to paraphrase Mark Twain 'the use of, rather than the best possible word, its second cousin'. 'Accept' is not the same as 'except'. 'Affect' and 'effect' are not interchangeable. 'Then' refers to time, 'than' refers to preference.

Also, watch out for phrases that don't exist: 'spoke to' does; 'asked to', and 'greeted to' do not (so don't write " 'Hello,' I greeted to him" or " 'What do you think you're doing?' I asked to her." Yes, the latter is based on a real example I've seen). 'Talked to' needs an 'about' at the end (as in, "I talked to my cousin about our current predicament with the zombies"). I'd make some general guidelines, but unique phrases do sometimes work and are best handled on a case-by-case basis. If you're reviewing and something doesn't quite read right to you, point it out.

For writers, to be sure the word you're using is the one you want, look it up a thesaurus (in Microsoft Word, you highlight it and right-click, then choose 'synonyms'. In Works, highlight it and go to the thesaurus under 'tools' on the toolbar. Failing that, use a paper thesaurus). If the synonyms match what you want, it's the right word. You can also use a dictionary; I prefer to use the thesaurus on MS Word because it's right there as I work. If the thesaurus has no matches, then I go to or haul out my mom's college dictionary from its convenient place on the top shelf (if your paper dictionary weighs more than three pounds, keep it somewhere close to the ground. Better for your back that way). A Google search may also help you figure out if your phrasing is just obscure or downright unknown.

And finally, remember that not only should your word mean what you think it means, it should also be fitting with your tone, setting, and the vocabulary of your POV character. Avoid modern phrases if you aren't writing in modern times. Avoid using casual or quaint words in emotionally charged scenes. Don't use highly scientific words if your character isn't a nuclear physicist, etc.

Last but not least, good luck with your writing!