I've only been to Costa Rica once, so I can't offer much in the way of tips, but I second the idea of using eBird. The new hotspot tool can be quite useful, but there are a couple of older methods I've used to prepare for trips as well: search for a common species in the area you will be visiting, click on the points, and then open up some checklists. The hotspot tool has mostly replaced this method, but it can still help if there aren't many identified hotspots where you're going.
The other good tool is the email alerts. Sign up for daily rare bird summaries for the area.
Wherever you go there, you are bound to see some great birds!
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