dpkg --get-selections > selected-packages.txt
dpkg --set-selections < selected-packages.txt
I've long used PkgBackup. I do not recommend relying on the Dropbox integration; I've had problems with it in the past. If you use it, you should also periodically email your backup to yourself.
Interesting. There was an update to PkgBackup overnight and now even though I have backups listed on Dropbox it would let me use them, saying no backup exists. Hoping Jibril has a fix for this soon as the Dropbox has worked well for me in the past.
What do you recommend for your backup facility?