All things being equal, fewer fields on a form will generally result in a better conversion rate. Imagescape did a study where they found reducing a form from 11 fields to 4 fields resulted in a 120% increase in their conversion rate. That is more than double. In another study, reducing a form from 9 fields to 5 fields resulted in a 34% boost in the conversion rate.
How many fields are on your online giving or online event registration form? Do you need them all? Are they always necessary? For example, I understand needing a large number of fields for the primary giving form for your site since anyone could be visiting it to make a gift. However, what about an email solicitation to a specific group of prospects? You should already know a lot of information about them. Could you use a form with fewer fields? If you know you are soliciting alumni, do you really need a class year field?
Related to fewer fields is also REQUIRING fewer fields. Some studies have shown removing a phone number resulted in a 5% increase in conversions. Furthermore, removing the requirement for a street address can boost conversions by 3%. If you always require a phone and address now, do you need to? For example, what if it is a free event?