Briefs should be brief
One thing I've learned over the years is appellate judges have increasingly less time to read briefs, as caseloads have increased while the number of judgeships on each court have by and large remained the same. That means you're going to have to continually edit your brief, perhaps four or five times, and eschew points that don't really advance your argument. Don't think you have to impress your client by writing to the page-limit maximum. Make your main points stand out.
Write in short sentences
Run-on sentences with too many clauses and subclauses are difficult to read, and your argument will have less impact. This goes back to the point that judges have less time to read your brief, let alone try and understand your argument. Break up sentences into short declarative statements and avoid the use of passive voice. Also, don't be afraid to use commas! Commas are very important tools of understanding. As an example, consider the first sentence in this segment. It's easier to read with a comma between "read" and "and."
Try and come up with a creative angle before you write a word
Avoid hackneyed assignments of error which don't differentiate your case from dozens of others, like, "The judgment is against the manifest weight of the evidence." Instead say, "The trial court erred in awarding only $500 in damages when the evidence was undisputed that appellee has not paid anything toward his court-ordered reimbursement since 2009." Or, instead of "Appellant was denied the effective assistance of counsel," get to the thrust of how your client didn't get a fair shake by saying, "The defendant was denied the effective assistance of counsel when his lawyer advised that he reject the state's plea bargain, which would have resulted in a more favorable sentence." In other words, write arguments that will captivate the judges' interest and make them actually want to read your brief.
Don't include too many assignments of error
Unless it's a death penalty case, try and winnow your arguments into two or three, occasionally four, assignments of error at most. Appellate judges have told me in countless seminars that their ability to focus on a really good argument---perhaps your only good argument---is diluted by tangential arguments. Tell your client this reality, and ask if a longshot argument really should be included. Also, many appellate judges are skeptical of, say, 10 or 12 assignments of error, which may even come off as a personal attack of the trial judge. My personal guide is, if I'm not comfortable talking about it at oral argument, it won't be included in the brief. Another approach is to combine arguments into a single assignment of error.
Finally, use proper citation form
This issue is more important than you realize. Don't make the mistake that proper citation form is only for law clerks and geeks. Law clerks themselves have told me how the briefwriter's credibility is compromised when proper citation form isn't followed. I'm not talking about typos, but when the briefwriter obviously shows an ignorance of citation form. It's not as bad as misspelled words or bad grammar, but close. And you certainly don't want your improper citations to be a distraction from your substantive arguments.