I'd suggest that we try modifying the page title (the one that shows up at the top of the browser window) to contain the most important keywords. Currently, the common part reads just "- IMSLP", so for example the title of the Main Page is "Main Page - IMSLP". This certainly is not something one would search for in Google, unless (s)he knew the site already (and happened to actually remember the name.....)
I think the title should contain at least some of the following keywords:
- sheet music
- (music) score(s)
- free
- classical (music) ?
- public domain ?
- download(s) ?