If you have a form with just one amount specified then we can make this the default fairly easily: a simple bit of JavaScript will do it for 98%+ of people.
Put this in an unformatted text part below the form:
<script type="text/javascript"> $('.DonationFormTable input[type=radio]:first').click(); </script>
Try copy and paste and hopefully it should work!
Multiple amounts
If you have four giving amounts listed it us possible to have it select a specific option using the code below. For example, $25, $50, $75, and $100 with a page where $75 is auto-selected.
<script type="text/javascript"> $(document).ready(function() { $('.DonationFieldControlCell span:contains("75"):first').closest('tr').find('input[type=radio]').click(); }); </script>