http://shine.yahoo.com/financially-fit/wedding-costs-pay-013200748.html "If you're getting married and unsure if, or how, to broach this topic with your family, here's some etiquette. First, don't assume you're in it alone. If your family doesn't reach out to you first about contributing to your wedding, it doesn't mean they're not interested. The only way to find out is to ask politely."
I'm fairly certain there is no polite way to ask for money for your wedding. My favorite line is this one though:
" 'I felt it was a smart move to put our pride aside and ask our parents for help," says Katherine. "We wouldn't have had the wedding that we had.' "
Then maybe you should have planned a wedding you could actually afford. What a novel idea!