Mistake #1 - Not Doing the Research Up Front
You need to know what legal issues are involved in your problem - not only so that you can ask the right questions when interviewing lawyers, but also so you'll know what group of lawyers to start your search with. Use Avvo Answers, Legal Guides or other online resources to quickly narrow down your legal issues to a particular specialty.
Mistake #2 - Not Checking for Lawyer Discipline
Has you lawyer been disciplined by the bar? Sure, most haven't, but you don't want to make a mistake here. While a minor infraction years ago may be explainable, more recent, serious or repeated violations are major red flags. Attorneys are typically disciplined for things like co-mingling client funds or failing to adequately represent clients. And, while past results are no guarantee of future performance, you don't want to experience these problems yourself if you can avoid it. You can easily check disciplinary history on a lawyer's Avvo profile, or check with your state's licensing authority - some of them make disciplinary history available online; in other states you have to call or write.
Mistake #3 - Not Interviewing Your Leading Contenders
Many consumers go with the first lawyer they talk to. While that might work out, given the stakes of most legal matters you owe it to yourself to talk to at least a few lawyers to find one that has the right blend of competence and approach to your issues. Everyone is different, and you need a lawyer who will mesh well with your personality and expectations about how the case will be handled. Take the time to interview a few lawyers before signing with one.
Mistake #4 - Not Knowing What Questions to Ask
So you take my advice and interview a few lawyers - what do you say? FInd out the extent to which the lawyer has handled cases like yours. Get answers on results (and watch for evasive responses). Ask about approach to your matter - if you need something handled sensitively, fast, or hyper-aggressively, make sure your attorney matches the style you need (you don't want to hire a pit bull if you're looking to get divorced amicably, for example). Finally, make sure you are in synch on cost expectations and how quickly - and in what form (e-mail, phone, letter, etc) - you and your attorney will communicate.
Mistake #5 - Not Checking References
Even if you've gotten a stellar referral on an attorney, and even if you've interviewed five and chosen a clear winner, do yourself a favor and get a community view. See if there are client reviews to be read online, or ask the attorney for client references. You can also talk to others in the legal community and get a sense of the lawyer's reputation. Like attorney discipline, in most cases things will check out fine. However, you need to do this simple piece of diligence to avoid unfriendly surprises. The slick charmer you hired just may turn out to be a lazybones with a reputation for sleeping through trial. You won't know unless you ask around.