Be sure to also check out the resources on the Primary & Historical Sources page for additional score and manuscript collections.
Finding vocal music:
If you're looking for a particular aria, song from a musical, pop song, etc., search for the song title using a keyword search rather than a title search.
Finding instrumental music:
Start with a keyword search rather than a title search, and include any unique identifiers such as an opus or Köchel number.
Use Grove Music Online to identify the definitive title of the work you're looking for. Every entry for an individual composer includes a list of that composer's complete works (in the entry, look at the bottom of the left sidebar for Works). This is where you can find the most authoritative version of the title for a given work.
Use Historical Sets, Collected Editions, and Monuments of Music, by Anna Harriet Heyer, to find music published in anthologies.