First, take note of every one of the early arrivers that you can. Species that fall into this category include Louisiana Waterthrush, Hermit Thrush, Purple Finch, Blue-headed Vireo, several species of woodpeckers, and White-breasted Nuthatch like this one trying to hide in my tray feeder.
Second, try to learn their calls. There have been years when we've only gotten Hermit Thrush because we knew where to listen, and we knew their "chuck" notes or their Catbirdesque whine.
Finally, unmated or closely opposing males are your friends. We easily nailed the nuthatch one year because we had found two pairs nesting very close to one another, and the males were constantly yammering at each other to keep to their own side of the line. Several teams missed this species that year, and these were the only ones we heard.
If you won't be scouting all week and there are some species you are worried about, don't hesitate to ask where some of our most reliable birds are. The sharing ethic runs strong these days, and there are plenty of people willing to pass on what they've found.