1. Radiology and Mountainbeauty make good points -- their binding in China is most important -- photos of other children in China, house property, bank savings in China, etc.
2. If you really want to increase the financial strength/proof at your side, you can send your parents these documents:
1) W2 and last year's tax return form;
2) Bank statements;
3) An official bank letter showing your balance (may charge $10-20);
4) I-134, Affidavit of Support (one form for one person).
Best wishes!!!