Get to know your greatest asset - your database.
If there is no history of major giving at your organisation - if you are starting from scratch, as it were - you would do well to consider getting your database screened for wealthy prospects.
Data screening companies will compare your database against their own database of wealthy or well connected people (company directors, trustees of grant giving bodies, shareholders, etc.) and pull out the wealthy. They can also check and update invalid names & addresses, phone numbers, etc. and select people by specific criteria (age, location, etc.)
Here are some data screening companies (if you know of any others, please contact me and I shall add them):
Fundraising Research & Consultancy Ltd
Screening costs money, of course, so shop around to get the best deal. Ask them who they have worked with (or get the information from their website, for those that have it) and then contact the organisations in question and ask them what they thought of the screening company.
And if you do not have the money to pay for data screening, you can always take the DIY approach, as I have done before now. Best to get yourself comfortable for this option, though, as it can take some time.
Get yourself a copy of the latest Sunday Times Rich List, and go through the entrants one by one to see if they are on your database. Laborious? Yes. A waste if time? No.
Not, that is, if you only search for the more unusually named people on the list (which, let's face it, is actually quite a few of them). By doing this you avoid all the time and effort that will need to be spent to check if the David Wilson on your database is the David Wilson (worth £400m) or some other David Wilson (worth 400 yen). But how many people will there be on your database called Hamish Ogston, Thor Bjorgolfsson or Kirsten Rausing? If one of these comes up, it's a fair bet they're the one and only, and it should not be too hard to confirm this, if you have a home or work address (as you should do, if they've given you some dosh).