Since your question was very open ended I will answer in two ways.
- I create a list of common and obvious campaign-level negative keywords and add them via Google AdWords Editor. I use keyword tools like AdWords Keyword Tool, Keyword Discovery and WordTracker to find as many as possible.
- Next, I monitor my phrase match keywords for any phrases that include unwanted terms and add those to an Ad group or campaign level whichever is more appropriate.
I save a text file list of all campaign keywords including all the campaign-level negative keywords. These files are helpful to review when setting up new campaigns.