It's a hard life, being a birder. I have a particular thing for waders, which can be some of the trickiest birds to identify in New Zealand, and we're at the easier end of the scale! Yes, identification of this group of birds can be quite the challenge, and so here's a few tips on how to identify the oddity from your normality.
Familiarize yourself on the species which may hide something special. As an example, Great Knots often associate with the abundant Lesser Knot, so knowing as much as you can about Lesser Knots is a good place to start. Lesser Knots are highly variable, often with variable markings on their sides, variable bill length and variable leg colouration depending on age, subspecies and individual markings. So learning about all the ranges in Lesser Knot will make the Great Knot more obvious when you next bump into them.
This technique is particularly important with terns, which all look extremely similar, Knowing the difference in leg length, bill colour and crown shape of the most abundant species (e.g. White- and Black-Fronted Terns, Caspian Tern) is crucial, Often you will only be able to identify the unusual species, including the rather ironically named Common Tern, by a slight difference which takes an experienced eye to notice.
Stick on it for a bit. Take a few notes. It'll all help you in the future finding something special, as you have a basis on what features occur in the more common species.
So there you go. Some tips on how to get used to more unusual species of birds by becoming familiar with the most common. The best species to start with is a thrush and juvenile, immature and female blackbirds, who are found in gardens almost nationwide. Watch them, and notice every difference you can between the species, and with other birds of their own species. Soon you'll be able to tell the difference in the blink of an eye.
Image of a Broad-Billed Sandpiper at Kidd's Shellbank by George Hobson