A quick search didn't reveal this, so I thought I'd pass it on.
If you've got some really tough stains / burned-on crud that you seemingly just can't shift, try this:
To a reasonably dry (and presumably dirty) flask, pour in a small handful of table salt (ordinary NaCl), along with a solvent like acetone or toluene (whatever you think the crud is most soluble in). Swirl swirl swirl. The salt acts as a gentle abrasive removing the crud, the solvent helps the process. This is my trump card if nothing else works, it has worked every time (sometimes requires more effort than other times).
At least, I *think* the salt is a gentle abrasive, I haven't had any flasks cleaned this way implode under vacuum *yet*.