When my bike budget gets down, I trade only. I've learned from wrk101 and others to trade first, persevere in trading, with patience, and buy as a last resort.
When my budget has been to zero, I take a hard look at my bikes, and ask these questions, per bike:
A. Do I absolutely need it? (Always start with the "no," and work up.)
B. When was the last time I rode it?
C. Is it "specialized," like a triple or a full-on race bike?
D. How unique? There are still probably hundreds of Ironman bikes out there, which hurts to admit.
E. Can I recoup my investment? Generally no, but parted out, it may be pretty darn close.
F. What's left? Can I still ride centuries, tri's, hills?
This generally allows me to sell at least one if I need to, without regret. I pretty much know exactly which bikes I'd simply not part with, and everything else is expendable, with varying degrees of baggage attached.
They are, gasp, just bikes. I really only need one to ride.
My dog, my son, my wife, those are real. OK, not in that order...
Maybe not always as fun, but real.