According to the Internet Advertising Bureau, the average
Abandonment Rate for shopping carts is 41%.  On many sites it is
much worse, especially for contact forms. If you’ve never
examined the Abandonment Rate for your forms, don’t be surprised
if you find it is over 90%.  You can easily calculate your
Abandonment Rate for a form by comparing the number of views of the
form with the number of views of the Thank You page, or whatever page
it is the visitor gets when they submit the form.  If you have no
discrete, measurable page, which is served when someone fills in a
form, then change the site design so you have something to measure.

The basic rule for reducing abandonment on forms is to ask fewer
questions.  Many people treat contact forms as an opportunity to
engage in some market research.  They may ask questions like “how
much is your budget?” or “where did you find our site?”  Each of
these questions will be a reason for someone not to complete the
forms.  Remember what the form is for – to get the contact
information from a potential customer so your sales team can start
talking to them.  Is losing a potential customer a reasonable
price for a little market research?

For sales systems with abandonment issues, the lesson is to keep
selling.  Don’t assume that once a visitor has put something
in the shopping basket they’re committed to buying it. 
They’re only committed when you’ve got their money. 
Many people have second thoughts about buying a product once
they’re asked to put in their credit card information.  This
is the time to remind them what they’re going to get for their
commitment, why they liked the product in the first place.  Some
shopping sites in the USA go one step further.  If you attempt to
leave their sites from the credit card payment page, they’ll
offer you a discount on the spot.