I raised around $2,500 last year for Tour de Cure, my highest ever. I know people who have raised much more than that and I bow to them, but I'll share what worked best for me.
1. One and only one spam email to everyone in your personal email contact list. Everyone. Tell them it's the only email you will send. Tell them what is motivating you. Tell them how far you will ride.
2. One and only one email to people you know at work. Don't spam the company , just people you personally know. Again tell them why, how far, how you will train, etc.
3. Face to face, tell people you know what you are doing. Ask if they would consider donating just a little bit. Have literature in-hand with your online donation page if they have one.
4. Find some personal motivation to raise way more than the minimum. Personally, I got into a competition with my boss to raise the most money. We stared under at around $500 and both ended up over $2,000. I eventually contacted my next level boss, his boss, the President of the company, and other executives. I actually ended up getting a commendation for getting the executives involved.
It took a little effort to get over the discomfort of begging for money. But once I got over that and started seeing the real generosity of people, it became easy.