jakebohall - 5:22 pm on May 23, 2013 (gmt 0)
Take a look at year over year data specifically looking at traffic sources. Is there one particular source (organic, paid, referral, direct) that seems to be dropping?
If you notice the drop is in referral traffic, you may have simply lost a link from a good referrer.
If you notice your drop is in direct traffic, maybe your brand has lost recognition in your space.
If you notice your drop is in paid search, did you decrease adspend? Is there more competition?
If you notice your drop is in organic, then try to identify the specific keywords causing the drop. Did you lose rankings for those terms? Is traffic what it used to be (look at Google Trends data)? If it looks like you are losing across a bunch of terms, try to identify any specific date that the traffic started dropping and then cross-reference that against a list of known algorithm updates to see if there is a correlation.
Good luck! Let us know what you find and we may be able to help you drill down a bit further.