Most think you can run a pentatonic scale, mode, or pattern of some sort and call it a solo.
Lead breaks provide melody and should be thought of as such. Good luck this does not come quickly.
I found this video hits it on the head.
Copy down the hints that appear on the screen. Pay attention to his right hand. The left hand is a great example of how one bass line riff can be used the entire song.
http://www.youtube.com/watch?v=r0iZ1j00wSU
Notice the phrasing, any melody must have room to breath. A string of notes turns into noise quickly.
Who says you have to have an instrumental lead break? If you have a vocalist the vocalist is providing the melody, skip the instrumental lead solo, sing the chorus once after the 2nd verse and then again after the last verse, i.e. end with the chorus. It's not stamped in stone that you must have a instrumental solo. When your buddy is ready then put one in.