I know that there are plenty of you who are bound to completely ignore my earlier advice about not putting songs in your spec scripts. Maybe you don't care about looking like an amateur, maybe you don't care that many of those songs are expensive and maybe you don't care about setting off a complicated approval process from whoever holds the rights to those songs.
Come to think of it, if you're going to ignore all those reasons I'm guessing you'll ignore my tips below. And yet I still spit into the wind.
...ever have one character tell another character that a song/band will "change their life." Especially as a bonding/romantic scene between the two.
... use U2 in your musical montage. They seem to be the go-to band for depressing romantic montages ("With or Without You") and moments of ecstasy ("Beautiful Day.") If I'd kept a count on the most used band in the specs I've read, they'd easily top the list. Be original.
... use a song in the exact same way another movie has. And if you're doing this on purpose for parody purposes, the joke had better be DAMN funny.
... use songs that have been overdone by other films, either for comedic or serious purposes. Overused songs include: I Will Survive, YMCA, The Pina Colata Song, Walking on Sunshine
... specify shot-by-shot what happens at each stanza of the song. It's really annoying and makes it hard to go with the whole, "well, I'm sure they can just replace this song if they don't get clearance."
... have a character start to sing a song and then collapse into an overwrought breakdown.
... get carried away with a pretentious music video in the middle of your extremely incomprehensible screenplay.
... use unexpected songs, even from familiar artists. Cameron Crowe is the master of this. In Almost Famous he used two Elton John songs: Tiny Dancer and Mona Lisas and Mad Hatters. The former is a decently well-known hit, but wasn't heard regularly on the radio nor had it been used in any movies. The latter is a rather obscure song. In Jerry Maguire, when Crowe used Springsteen, he went for Secret Garden rather than any of his far better known hits.
... know the value of a totally off-beat song choice. Who would have thought that Queen's Don't Stop Me Now would have been so right for Shaun of the Dead's bar fight?