From an essay by Bousoño I first learned of a technique used by Jaime Gil de Biedma and Guillermo Carnero. The normal Spanish alejandrino has two equal halves of 7 + 7. What Gil de Biedma does is take away one syllable from the line:

"Después de la muerte / de Jaime Gil de Biedma" 6+7

"un poco de dulzura / aunque no lo creo" 7+6

"Juan, María Rosa, / Marcelino, Joaquina" 6+7

"y el frío repentino / de final de agosto" 7+6

You still have the balanced phrases, but there is a little hiccup in the line. When you combine these lines with 11s, 9s, and regular alejandrinos, you really get a varied meter. So varied that is is almost indistinguishable from prose itself!