Tailor letters for specific newspapers. If you're going to write to the Wall Street Journal you can be pedantic, but you'd better dumb it down for certain papers. You're always better off preempting editors. Use an organizational affiliation if possible. Consider starting a local drug policy organization, if only to get the reform message out.
Most newspapers require that letters submitted be exclusive.
When recycling letters make slight edits. Having the exact same letter published in both the Chicago Tribune and the Chicago Sun on the same day would not be good. Having the exact same letter simultaneously published in Alabama and Idaho should not pose a problem. It's important to keep track of when you last sent a particular newspaper a letter. Likewise, avoid sending the same letter to the same newspaper twice.