This sounds kind of backwards, but I would buy the Garrigues and Dean field guide and figure out which species you would really like to see (you won't be able to see all the birds you want to--we're just finishing up our 8th trip and I've only seen 1/2 the species in CR that are found there) and use their range maps as a rough guide.
Also, it helps to have some idea as to how much time, how much money you can spend as well as what kind of weather you're willing to put up with (hot, dry, cold, wet), what aspects of travel (creature comforts, service, restaurants/food, swimming pool/beach) matter to you and especially any non-birding companions.
One general piece of advice regardless of the above is to stay at hotels set in good birding habitat, I.e. not in city centers or on the beach but rather in the forest. Some of my best CR bird sightings (e.g King Vulture, Ornate Hawk-Eagle, Emerald Tanager, Tody Motmot, Turquoise Cotinga, Great Green Macaw, White-tipped Sicklebill, Black Crested and White Crestec Coquettes ) have been on the grounds of lodges we've been staying at. There are a lot of lodges that cater specifically to birders but there many others that have equally good habitat.
Edited: 5 years ago