Same as most people have posted- I go without 
other things- new clothes, anime, books, lunch...
I still spend too much money on cels, but I will 
sometimes sell cels in my collection to buy 
others.  This has kind of a nice double feature- 
it gives me money toward a new cel I want, and 
gives other collectors a chance at a cel they 
might want.  I made a deal with myself and my 
husband when I started collecting- I will not 
collect more cels than will fit in one cel book. 
(and no, nothing sneaky like stacking them on top 
of each other *g*). 
The other answer to your question is- I don't.  I 
don't spend more than about $200 on a cel, I have 
only spent more than that one time.  I have 
*tried* a couple of other times, but usually 
couldn't even place a bid because some other 
person already put in a giant one on YJ for 
example.  Once in a while I get very lucky and 
can do a trade or partial trade for a cel I 
cannot otherwise afford.  Anymore, with most FY 
cels, I simply go without.  
Even after all this, I realize I must be addicted 
to cel collecting, because of the cels I go 
withough some things I really don't want to... I 
always think just one or two more- except that 
that happens weekly.  :D