One of the most important tasks in a PPC account is to check which searches your ad is displayed. Google Adwords and Bing Ads will default to the keyword match type to broad. What this means is that if you are advertising for the keyword "lawyer," your ad will not only show for "lawyer," but any other search Google or Bing thinks is related. This could be useful if, for example, your ads show up for searches for "attorney" or "DUI lawyer". But if it starts showing up for "Law and Order reruns" you are going to be paying a lot of money for a totally irrelevant click. Here are the two most important ways to combat this:
1) Bid low on broad match, high on exact match keywords. Exact match keywords will only show your ads on that exact search (or plural/singular forms of the search). Bid high for those keywords. You should still keep broad-match keywords (unless you have a very small budget) because broad-match will capture phrasing and synonyms that you don't have in your keyword list, but at a much lower bid because these keywords are less likely to be relevant. You can also add modified broad match and phrase match keywords at bids higher than your broad match keywords and lower than your exact match keywords.
2) Check your search term report at least once a month. Google and Bing will show you the searches it matched up to your broad match keywords. At the end of each month (or more frequently if you have the time), you can look at this list and see which irrelevant searches your ads have been showing for. You can then block those with negative keywords such negative phrase match keywords for "Law and Order" and "tv shows".
By simply doing these two tasks you'll be able to reduce the amount of irrelevant clicks you get and increase your conversion rates. This is especially important for lawyers as they have the highest cost-per-clicks of any industry. You don't want to pay $50 to refer a person looking for an old TV show to your attorney website.