I've actually been thinking about this question 
all day (sorry off from school, I've got the free 
time).

Personally, I don't commit to dibs at all for the 
one and only reason that I have an absolutely 
horrendous memory.  Unless it's someone I know 
well (and know that they collect a particular 
series or character), I'm not going to remember 
that they asked about a cel.  So instead of 
getting myself in a tangle, I usually tell people 
to 'feel free to ask again' or if it's an ebay 
auction to 'feel free to contact me at the end of 
the auction.'  Because if the person is serious 
about the cel, they WILL contact you.

Also, how many of you had 'dibs' people contact 
you about a cel, you put it on hold, and then 
play the waiting game for payment, only to have 
to pass on other people who you know will 
following through with payment?