As mentioned in earlier posts, I sometimes use the bloodline levels from Unearthed Arcana. I also disapprove of too much unnecessary cruft dragging characters down. Particularly in the case of the nearly-useless minor bloodlines - the only good reason to take them is RP.
So I use them slightly differently. In my game, each bloodline level isn't a level so much as it is a level adjustment. At certain intervals, instead of taking a bloodline level, you increase your level adjustment by one. It doesn't count as a class level for any purpose. However, because this is level adjustment, it can be reduced like any level adjustment.
Say you have a minor bloodline. When you ding 12th level, you need to increase your level adjustment instead of gaining a class level. But, since minor bloodlines count as a +1 LA, you may then spend 3,000, 6,000, or 11,000 experience (depending which LA reduction system you're using) to pay it off.
If you have an intermediate bloodline, then when you ding 6th and 12th levels, you need to take a level adjustment. At any time, you may reduce these level adjustments as if you were an LA+2 creature. Same goes for major bloodlines, except you gain LA at 3rd, 6th, and 12th level, and you may reduce them as if you were an LA+3 creature.
You may not, however, ever reduce your LA below +0.
I also allow monster classes, as from Savage Species, Libris Mortis, and some of the Races of books.
Monster classes for creatures that have level adjustment include empty levels, where your effective character level increases and you gain some powers, but you gain no hit dice or skill points. HeroForge even represents these empty levels straightforwardly as level adjustment.
I allow these, too, to be reduced as level adjustment. The experience cost is determined by the total number of empty levels of the monster class, which is to say, the total LA of the final creature.
For example, take the myconid monster class I posted some time ago. The full progression includes 6 HD and 6 empty levels. Put another way, a full-power myconid sovereign has 6 HD and +6 LA. At any point, a myconid character may pay off its empty levels, reducing its ECL but losing no abilities, as if it were already a +6 LA creature.
Again, you may never reduce your LA below +0.