What sort of fluffy do you want? open, bouncy yarn is fun; for this I spin the singles without a whole lot of twist in it (enough twist so that it holds together when I tug, plus a little bit) -- I use a "ply" test where I let a length fold back on itself. If your singles are fairly low twist, then the ply test will be _very_ poufy indeed, as the singles open up again with the reverse-twist of the ply (which is the desired result!)
Also starting with a wool with alot of crimp helps add bounce to yarn -- CVM/Romeldale, Cormo, Targhee are among my favorites for bounce. I'm sure others can list more!
Another sort of fun fluffy yarn is to spin from washed locks of some curly-hair style wool -- kid Mohair or adult Mohair, English Leicester, Wensleydale, Cotswold all have this trait, of looking like ringlets in the washed locks. I open up the locks a bit at the base, but as I'm spinning let the tip-end run "free" of the yarn itself. This gives a yarn full of loose curls as a result. I like to ply this yarn with thread or with itself -- the thread doesn't tend to grab at the curls too much, but gives the single a chance to open up a little bit itself. Plying with itself usually gives a fairly bulky yarn (since I spin singles for this fairly "big" to begin with -- about DK or worsted weight, if you ignored the curly bits).
(posted by me this day on spindlers)