Try not to use popular music.
It takes forever to get clearance, and costs a fortune!
Often the music companies such as Sony BMG or EMI only give you a short license for educational purposes, but if you want it on youtube or shown at Top Screens, that won't work because you're not allowed to use it for them and you could get sued, and you don't want that!
If you don't get copyright clearance you can NOT show it at Top Screens!
And that would be a shame because if it's a good film you should want it to be able to be shown!
I was fortunate enough to have a band that I have friends in, so I used their songs and it fits perfectly! So if you know any good musicians or bands, ask them nicely and they may give you copyright clearance free of charge!
I suggest you look at production music libraries.
XTreme Music Director's Cuts are very good and you don't have to pay much at all. I used a track from this and licensing was easy and cost a little, but nothing compared to what the music companies such as BMG or EMI charge for you to use their music.
I also got clearance for a song which is very popular, "La Mer" by Charles Trenet which is an older song (I didn't use it in the end as it didn't fit the tone of my film).
After emailing the copyright people, they said I could use it free of charge and for as long as I like. I believe this was because it was a very old song and the artist is dead, so it's possible that the copyright stuff had expired.